Laser diode and semiconductor light-emitting device producing visible-wavelength radiation

ABSTRACT

A laser diode includes a substrate having a lattice constant of GaAs or between GaAs and GaP, a first cladding layer of AlGaInP formed on the substrate, an active layer of GaInAsP formed on the first cladding layer, an etching stopper layer of GaInP formed on the active layer, a pair of current-blocking regions of AlGaInP formed on the etching stopper layer so as to define a strip region therebetween, an optical waveguide layer of AlGaInP formed on the pair of current-blocking regions so as to cover the etching stopper layer in the stripe region, and a second cladding layer of AlGaInP formed on the optical waveguide layer, wherein the current-blocking regions having an Al content substantially identical with an Al content of the second cladding layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is based on Japanese priorityapplications No.11-220649 filed on Aug. 4, 1999, No.11-229794 filed onAug. 16, 1999, No.11-243745 filed on Aug. 30, 1999, No.11-339267 filedon Nov. 30, 1999, No.2000-057254 filed on Mar. 2, 2000, andNo.2000-144604 filed on May 12, 2000, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to semiconductor devicesand more particularly to semiconductor light-emitting devices and laserdiodes.

[0003] Particularly, the present invention relates to a laser diodeoperable in a wavelength range of 360-680 nm. Further, the presentinvention relates to a laser diode for use in optical recording andoptical reading of information or light-emitting display of information.Further, the present invention relates to a semiconductor light-emittingdevice based on a III-V compound semiconductor material.

[0004] Further, the present invention relates to a vertical-cavity laserdiode suitable for an optical source of optical recording and reading ofinformation or light-emitting display of information. The presentinvention further relates to an optical information recording apparatussuch as a xerographic image recording system or an optical system andoptical telecommunication system including an optical interconnectiondevice that uses a vertibal-cavity laser diode.

[0005] In these days, efforts are being made to develop a red-wavelengthlaser diode operable in the wavelength range of 630-680 nm as an opticalsource of optical disk recording apparatuses. Such an optical diskrecording apparatus includes a DVD (Digital Video Disk or DigitalVersatile Disk) player. The laser diode is used in such disk recordingapparatuses as the optical source for reading and/or writing ofinformation.

[0006] In order to increase the writing speed of information into theoptical disk in such optical disk devices, it is necessary to increasethe output power of the laser diode used therein.

[0007] Hereinafter, a brief review will be made on conventionalred-wavelength laser diodes.

[0008]FIG. 1 shows the cross-sectional diagram of a conventionalred-wavelength laser diode of an AlGaInP system disclosed in theJapanese Laid-Open Patent Publication 11-26880.

[0009] Referring to FIG. 1, a substrate 1 of n-type GaAs carries thereona buffer layer 2 of n-type GaAs, a cladding layer 3 n-type AlGaInP, aquantum well active layer 4 including therein alternate and repetitivestacking of an AlGaInP layer and a GaInP layer, a cladding layer 5 ofAlGaInP of low carrier concentration (2−6×10¹⁷ cm⁻³), and an etchingstopper layer 6 of p-type GaInP.

[0010] Further, there is provided a ridge structure 10 on a part of theetching stopper layer 6 wherein the ridge structure 10 includes acarrier-diffusion suppressing layer 7 of p-type AlGaInP, a claddinglayer 8 of p-type AlGaInP, and a band-discontinuity relaxation layer 9of p-type GaInP. Further, there are formed a pair of electric currentblocking regions 11 of n-type GaAs on the surface part of the etchingstopper layer 6 where the ridge structure 10 is not formed, and acontact layer 12 of p-type GaAs is formed continuously on the currentblocking regions 11 and the band-discontinuity relaxation layer 9therebetween. The contact layer 12 carries thereon a p-type electrode13, and an n-type electrode 14 is formed on the bottom surface of thesubstrate 1.

[0011] In the laser diode of FIG. 1, there occurs a current confinementin the ridge structure 10 wherein the ridge structure 10 provides acurrent path between the current-blocking regions 11, and the electriccurrent is confined into the ridge structure 10 thus formed of p-typeGaAs. Further, it should be noted that the current-blocking regions 11absorb the optical radiation from the quantum well active layer 4 andthere is induced a refractive-index difference between the ridgestructure 10 and the region outside the ridge structure 10 as a resultof such an optical absorption. Thereby, there occurs an opticalconfinement in the ridge structure 10.

[0012] Such a ridge structure 10, while being able to form so-calledoptical loss-guide structure in the laser diode, has a drawback in thatit increases the threshold current of laser oscillation due to theoptical absorption caused by the current-blocking regions 10.

[0013]FIG. 2 shows the cross-sectional structure of a red-wavelengthlaser diode disclosed in the Japanese Laid-Open Patent Publication9-172222.

[0014] Referring to FIG. 2, the laser diode is constructed on asubstrate 15 of n-type GaAs and includes a buffer layer 16 of n-typeGaAs, a cladding layer 17 of n-type AlGaInP, an active layer 18 ofGaInP, a cladding layer 19 of p-type AlGaInP and an intermediate layer20 of p-type GaInP, wherein the layers 16-20 are formed on the substrate15 consecutively by an epitaxial process.

[0015] In the intermediate layer 20, there are formed a pair of stripegrooves reaching the p-type cladding layer 19, and the stripe groovesthus formed define a stripe ridge 21 therebetween. Further,current-blocking regions 22 are formed by filling the stripe grooveswith a layer of n-type AlGaAs, and the entire structure is covered by acap layer 23 of p-type GaAs formed by an epitaxial process.

[0016] In the case of the laser diode of FIG. 2, the current-blockingregions 22 are formed of AlGaAs having a bandgap larger than a bandgapof the active layer 18. For example, the current-blocking regions 22 areformed to contain Al with a concentration of 39% in terms of atomicpercent when the laser diode is designed to operate at the wavelength of650 nm. In the case the laser diode is to be operated at the wavelengthof 630 nm, the Al content in the current-blocking regions 22 should be45% or more in terms of atomic percent. In such a case, thecurrent-blocking regions 22 are transparent to the laser beam and theloss at the optical waveguide is minimized.

[0017]FIG. 3 shows the cross-sectional diagram of a red-wavelength laserdiode disclosed in the Japanese Laid-Open Patent Publication 7-249838.

[0018] Referring to FIG. 3, the laser diode is constructed on asubstrate 24 of GaAs and includes, on the substrate 24, a cladding layer25 of n-type AlGaInP having a composition(Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P, an active layer 26 having a quantumwell structure formed by an AlGaInP barrier layer and a GaInP quantumwell layer, an inner cladding layer 27 of p-type AlGaInP having acomposition of (Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P, an etching stopperlayer 28 of p-type GaInP having a composition of Ga_(0.5)In_(0.5)P, anouter cladding layer 29 of p type AlGaInP having a composition(Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P, a buffer layer 30 of p-type GaInPhaving a composition of Ga_(0.5)In_(0.5)P, and a cap layer 31 of p-typeGaAs.

[0019] The laser diode is formed with a mesa structure by a wet etchingprocess, wherein the wet etching process is conducted while using an SiNmask formed on the cap layer 31 with a width of 6 μm, until the etchingstopper layer 28 is exposed. After the mesa structure is thus formed, apair of current-blocking regions 32 of n-type AlInP and a pair of capregions 33 of n-type GaAs are formed on the mesa surface. Thereby, thecurrent-blocking regions 32 are grown so as to have a composition ofAl0.5In0.5P on the part making contact with the mesa surface. Afterremoving the SiN mask, a contact layer 34 of p-type GaAs is formed so asto cover the cap regions 33, the current-blocking regions 32 and the caplayer 31 on the mesa structure.

[0020] In the laser diode of FIG. 3, too, the problem of waveguide lossis avoided due to the large bandgap energy of AlInP used for thecurrent-blocking regions 10. Further, the use of the AlInPcurrent-blocking regions 32 is advantageous in view of the fact thatAlInP has a smaller refractive-index as compared with the inner andouter cladding layers of p-type AlGaInP. Thereby, it should be notedthat there is formed a real refractive-index difference between theregion inside the ridge and the region outside the ridge, and a realrefractive-index waveguide is formed in the laser diode.

[0021] In the laser diode of FIGS. 2 and 3, it should be noted that thecurrent-blocking regions 22 or 32 contain an increased amount of Al forminimizing the optical absorption by the current-blocking regions. Asnoted already with reference to FIG. 2, the Al content in thecurrent-blocking region 22 of AlGaAs has to be set to 39% or more inatomic percent when the laser diode is to be operated at the wavelengthof 650 nm. In the case of the laser diode of FIG. 3, on the other hand,the current-blocking region 32 contains Al with an amount of 50% interms of atomic percent in the vicinity of the mesa surface, while thisvalue of Al concentration is larger than the Al concentration (35% inatomic percent) of the AlGaInP cladding layer typically used in anAlGaInP laser diode. When the Al content in a semiconductor layer islarge as such, there tends to occur a problem of optical damaging at theedge surface of the laser optical cavity due to non-opticalrecombination of carriers. It should be noted that the increase of Alcontent tends to increase surface states, while the surface states tendto facilitate the non-optical recombination of carriers.

[0022] Thus it is an object of the present invention to provide ared-wavelength laser diode having a reduced optical waveguide loss andsimultaneously a reduced optical damage at the edge surface of theoptical cavity formed in the laser diode.

[0023] As noted already, the laser diode of the AlGaInP system isbecoming an important target of investigation in relation to applicationto laser beam printers, optical disk drives and the like, due to thefact that the laser diode of this system can produce an optical beamwith the wavelength range of about 600 μm.

[0024] In the application to the optical source of disk drives, it isrequired that the fundamental mode of laser oscillation is a horizontallateral mode of single peak. Further, it is required that astigmatism issmall.

[0025] Such a single fundamental mode laser oscillation with reducedastigmatism is realized by using a real refractive-index waveguidestructure, and there is proposed a visible-wavelength laser diodestructure based on an AlGaInP system as represented in FIG. 4.

[0026] Referring to FIG. 4, the laser diode is constructed on asubstrate 42 of n-type GaAs and includes a cladding layer 43 of AlGaInP,an active layer 44 of GaInP, and a cladding layer 45 of AlGaInP, formedconsecutively on the substrate 42.

[0027] After forming the cladding layer 45, a ridge stripe of aninverse-mesa structure is formed so as to extend axially, andhigh-resistance regions 46 of AlInP are formed at both lateral sides ofthe stripe structure by causing a selective growth process while usingan SiO₂ mask on the stripe region. Further, a GaInP layer 48 and ap-type GaAs layer 49 are grown selectively and consecutively on theAlGaInP layer forming the stripe region while using an SiO₂ mask formedon the high-resistance regions 46. Further, n-type GaAs regions 47 areformed on the high-resistance regions 46 at both lateral sides of thecentral stripe region, and a p-type electrode of Cr/Au/Pt/Au structureis formed on the top surface of the p-type GaAs layer 49. Further, ann-type electrode 41 of AuGe/Ni is formed on the bottom surface of thesubstrate 42.

[0028] In such a structure, there is formed a real refractive-indexwaveguide structure in correspondence to the central ridge stripe.Generally, such a laser diode is fabricated such that the epitaxiallayers constituting the laser structure achieves a lattice fitting withthe GaAs substrate 42.

[0029] On the other hand, the Japanese Laid-Open Patent Publication5-41560 describes a refractive-index waveguide laser diode that uses adouble heterostructure of a mixed crystal of (AlGa)_(a)In_(1-a)P(0.51<a≦0.73) formed on a GaAs substrate, wherein the foregoing doubleheterostructure is formed with an intervening lattice misfit relaxationlayer having a composition represented as GaP_(x)As_(1-x).

[0030]FIG. 5 shows the relationship between the band edge energy and thelattice constant for various III-V crystals, wherein the continuouslines represent the band edge energy of the conduction band Ec and thevalence band Ev of a GaInP mixed crystal while the broken linesrepresent the conduction band energy and valence band energy of an AlInPmixed crystal.

[0031] Referring to FIG. 5, it can be seen that a mixed crystal of theAlGaInP system can be used for the cladding layer and the active layeras long as the AlGaInP mixed crystal has a composition in which thelattice constant is smaller than that of GaAs. When the composition ischosen as such, the bandgap energy increases and the laser oscillationwavelength shifts in the shorter wavelength direction. Thus, theforegoing Japanese Laid-Open Patent Publication 5-41560 proposes a laserdiode that can oscillate at the wavelength shorter than 600 nm, bychoosing the composition of the AlGaInP mixed crystal constituting thelaser diode.

[0032] On the other hand, the relationship of FIG. 5 also indicates thepossibility of improvement of performance of the red-wavelength laserdiode oscillating in the wavelength range of 600-660 nm, by using amixed crystal of AlGaInP having a lattice constant between those of GaAsand GaP, for the cladding layer and the optical waveguide layer.

[0033] Further, laser diodes having a refractive-index waveguidestructure with current-blocking regions of GaAs or AlInP are proposed.In such a refractive-index waveguide laser diode, it is also possible touse a mixed crystal of AlGaInP for the current-blocking regions.However, the use of a mixed crystal composition containing a largeamount of Al such as AlInP causes a problem to be described later.

[0034] In order to fabricate such a real refractive-index waveguidelaser diode, it is necessary to form a real refractive-index profile ina transverse direction of the active layer. Normally, this is achievedby forming a ridge-stripe structure or a groove-stripe during thefabrication process of the laser diode by an etching process and byforming a cladding layer or current-blocking regions of AlGaInP, and thelike, by a regrowth process.

[0035] In the case of forming a layer of AlGaInP on a substrate of GaP,GaAs or GaP_(0.4)As_(0.6) by an MOCVD process, there is a tendency ofextensive formation of hillock structure on the surface of the AlGaInPlayer thus grown when the AlGaInP layer is grown on the substrate havinga (100) principal surface or when the offset angle of the substrateprincipal surface from the (100) surface is small. This tendency ofhillock formation is enhanced when the mixed crystal layer thus growncontains a large amount of Al as in the case of an AlInP mixed crystal.

[0036] It is possible to suppress the hillock formation to some extentby using an offset substrate and by increasing the offset angle of thesubstrate. However, such suppressing of hillock formation by way ofusing an offset substrate tends to become difficult in the case of anAlGaInP mixed crystal containing a large amount of Al and Ga and hencehaving a lattice constant smaller than that of GaAs. Further, use of anoffset GaAsP substrate having a large offset angle poses a problem ofavailability as compared with the case of using a readily availableindustrial standard GaAs substrate.

[0037] When such hillock structure exists extensively in thesemiconductor layers constituting a laser diode or an LED, the deviceperformance or the yield of device production may be degraded seriously.This problem appears particularly serious in the case of regrowing amixed crystal containing Al. In such a case, realization of a sufficientcrystal quality is extremely difficult due to the surface oxidation ofthe underlying layer.

[0038] In the case of the laser diode disclosed in the JapaneseLaid-Open Patent Publication 5-41560, op. cit., it is believed thatfabrication of a satisfactory laser diode device with high-qualitycrystal layers is difficult.

[0039] Thus, it is an object of the present invention to provide a laserdiode operable in the wavelength range of 600-660 nm wherein the deviceperformance is improved by improving the quality of the crystalconstituting the current-blocking regions.

[0040] A material of the AlGaInP system is a direct-transition typeIII-V material having the largest bandgap energy except for a materialof the AlGaInN system. The bandgap energy can reach as much as 2.3 eV(540 nm in bandgap wavelength).

[0041] Thus, efforts have been made with regard to optical semiconductordevices of the AlGaInN system to provide a high-luminosity, green to redoptical source for use in various color display devices or a laser diodefor use in laser printers, compact disk drives, DVDs for optical writingof information.

[0042] In the case of a laser diode, a material system achieving alattice matching with a GaAs substrate has conventionally been used. Itshould be noted that a laser diode for high-density optical recording isrequired to produce a large optical output of short-wavelength in a hightemperature environment.

[0043] In order to construct a laser diode, it is necessary to provide astructure for confining both carriers and optical radiation in an activelayer or light-emitting layer by using a cladding layer. Thus, acladding layer is required to have a bandgap larger than a bandgap ofthe active layer.

[0044] In this regard, the material in the system of AlGaInP has adrawback in that the band discontinuity ΔEc on the conduction band tendsto become smaller. In such a case, the injected carriers easily escapefrom the active layer into the cladding layer by causing an overflow.When such an overflow of carriers takes place, the threshold current oflaser oscillation becomes sensitive with the operational temperature ofthe laser diode and the temperature characteristic of the laser diode isdeteriorated.

[0045] In order to overcome the problem, the Japanese Laid-Open PatentPublication 4-114486 proposes a structure that uses an MQB (multiplequantum barrier) structure, in which a large number of extremely thinlayers are stacked between the active layer and the cladding layer forcarrier confinement. This structure, however, is complex, and it hasbeen difficult to achieve the desired effect in view of the necessity ofprecision control of thickness of the layers to the degree of atomiclayer level.

[0046] In an ordinary edge-emission type red-wavelength laser diode thatuses a structure in which the active layer is sandwiched by a pair ofoptical guide layers having a composition represented as(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P, the desired optical confinement isrealized in the optical guide layers of the composition(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P. On the other hand, the optical guidelayers generally contain Al with a composition x of 0.5 or more, whilesuch a high concentration of Al in the optical waveguide layer causesthe problem of optical damaging at the optical cavity edge surface ofthe laser diode due to the recombination of carriers facilitated by Al.Thus, there has been a difficulty in obtaining a high optical outputpower or realizing a stable operation of the laser diode over a longperiod of time.

[0047] Summarizing above, conventional laser diodes constructed on aGaAs substrate with lattice matching therewith have a problem inoperation under high temperature environment, or high-output operation,or operation over a long period of time. For example, it has beendifficult to realize a red-wavelength laser diode operable under a hightemperature environment such as 80° C. with high output power such as 70mW or more, over a long period of time such as ten thousand hours. Thedifficulty increases with decreasing output wavelength of the laserdiode.

[0048] The material of the system of AlGaInP having a lattice constantsmaller than the lattice constant of GaAs is characterized by a widebandgap and is suitable for decreasing the output wavelength of thelaser diode or light-emitting diode. Thus, there is a proposal in theJapanese Laid-Open Patent Publication 8-18101 with regard to alight-emitting diode (LED) using the foregoing material system as wellas other material systems. Further, there are proposals of a shortwavelength laser diode oscillating at a wavelength of 600 nm or less.For example, the Japanese Laid-Open Patent Publication 5-41560 proposesa laser diode in which a double heterostructure having a composition of(AlGa)_(a)In_(1-a)P (0.51<a≦0.73) and a lattice constant intermediatebetween GaAs and GaP is provided on a GaAs substrate with an interveningbuffer layer of GaP_(x)As_(1-x) having a composition adjusted so as toachieve a lattice matching with the foregoing double heterostructure. Inthe foregoing proposal, the problem of lattice misfit is resolved byinterposing the buffer layer between the substrate and the doubleheterostructure.

[0049]FIG. 6 shows the relationship between the bandgap energy and thelattice constant for various III-V materials.

[0050] Referring to FIG. 6, the continuous lines represent thecomposition causing a direct-transition, while the broken linesrepresent the composition causing an indirect-transition. It should benoted that the material of the foregoing composition (AlGa)_(a)In_(1-a)P(0.51<a≦0.73) having the lattice constant between GaAs and GaP falls inthe region defined by the composition of AlInP and the composition ofGaInP. By using the material system of AlGaInP having a bandgap largerthan the bandgap of the material achieving a lattice matching with aGaAs substrate for the active layer and the cladding layers, it ispossible to reduce the oscillation wavelength of the laser diode to besmaller than 600 nm.

[0051]FIG. 7 shows the construction of a laser diode having arefractive-index waveguide disclosed in the Japanese Laid-Open PatentPublication 5-41560, wherein the laser diode has a lattice constantbetween GaAs and GaP.

[0052] Referring to FIG. 7, the laser diode is constructed on asubstrate 51 of n-type GaAs and includes a graded layer 52 of n-typeGaPAs formed on the substrate 51, and a superlattice layer 53 of n-typeGa_(0.7)In_(0.3)P/(Al_(0.7)Ga_(0.3))_(0.7)In_(0.3)P formed on the gradedlayer 52, wherein the substrate 51, the graded layer 52 and thesuperlattice layer 53 form together a GaPAs semiconductor substrate 54.The GaPAs semiconductor substrate 54 thus formed carries thereonconsecutively an optical waveguide layer 55 of n-type AlGaInP having acomposition of (Al_(0.7)Ga_(0.3))_(0.7)In_(0.3)P, an active layer 56 ofundoped GaInP having a composition of Ga_(0.7)In_(0.3)P, and an opticalwaveguide layer 57 of p-type AlGaInP having a composition of(Al_(0.7)Ga_(0.3))_(0.7)In_(0.3)P, and a first buffer layer 58 of p-typeGaInP having a composition of Ga_(0.7)In_(0.3)P is provided further onthe optical waveguide layer 57.

[0053] The first buffer layer 58 and the underlying optical waveguidelayer 57 are then subjected to a mesa etching process to form a ridgestripe structure, wherein the mesa etching process is conducted suchthat the optical waveguide layer 57 is left with a thickness of 0.2-0.4μm outside the ridge stripe structure.

[0054] At both lateral sides of the ridge stripe structure, a pair ofcurrent-blocking regions 59 of n-type GaInP having a composition ofGa_(0.7)In_(0.3)P are formed by a regrowth process, wherein thecurrent-blocking regions 59 function also as an optical absorptionregion. Further, a contact layer 60 of p-type GaInP having a compositionof Ga_(0.7)In_(0.3)P is formed on the current-blocking regions 59including the ridge stripe region formed therebetween, by a regrowthprocess. Further, p-type electrode 62 and an n-type electrode 61 areformed respectively on the top surface of the contact layer 60 and onthe bottom surface of the GaAs substrate 51.

[0055] In the foregoing laser diode that uses a material system having alattice constant between GaP and GaAs, it is necessary to carry outthree regrowth process steps, one for growing the GaInP buffer layer 58,one for growing the current-blocking regions 59, and one for growing thecontact layer 60. Thereby, the fabrication process of the laser diode iscomplex and the yield of production tends to be reduced.

[0056] In order to facilitate the fabrication of a ridge-waveguide laserdiode, there is also a proposal in the Japanese Laid-Open PatentPublication 10-4239, to form the current-blocking regions by way ofoxidation of an AlGaAs mixed crystal having a composition represented asAl_(x)Ga_(1-x)As (0.8<x≦1). According to the foregoing proposal, theridge structure is formed to have a width of 4 μm at the bottom partthereof, and there is provided a current path region as a non-oxidizedpart of the AlGaAs region of the foregoing composition ofAl_(x)Ga_(1-x)As (0.8<x≦1), with a width of 3 μm.

[0057] According to the foregoing proposal, it is possible to form alaser diode having the current-blocking structure in a single crystalgrowth process.

[0058] On the other hand, the laser diode of the foregoing prior art hasa drawback, in view of the difference in the lattice constant betweenthe material system having a lattice constant between GaAs and GaP andthe foregoing AlGaAs mixed crystal of the composition Al_(x)Ga_(1-x)As(0.8<x≦1), which achieves a lattice matching with the GaAs substrate, inthat the thickness of the AlGaAs mixed crystal layer of the compositionAl_(x)Ga_(1-x)As (0.8<x≦1) is inevitably limited when the AlGaAs mixedcrystal layer is to be provided in the material system having a latticeconstant between GaAs and GaP. Further, in view of the fact that thecurrent path region of the not-oxidized Al_(x)Ga_(1-x)As (0.8<x≦1) mixedcrystal layer extends such that the edge of the current path region islocated near the edge of the ridge structure, there appears asubstantial optical waveguide loss and increase of optical output poweris difficult.

[0059] Thus, the present invention has an object to provide asemiconductor light-emitting device formed of a semiconductor materialhaving a lattice constant between GaP and GaAs wherein the fabricationprocess is simplified. Further, the present invention has an object toprovide a semiconductor light-emitting device formed of a semiconductormaterial having a lattice constant between GaP and GaAs wherein theoptical waveguide loss is minimized and suitable for increasing outputoptical power.

[0060] Meanwhile, vertical-cavity laser diodes, which emit optical beamin a direction perpendicular to a substrate surface, draw attention inrelation to application of red-wavelength optical source in thewavelength range of 630-650 nm for use in high-density optical diskdrives and laser printers, in view of the fact that a vertical-cavitylaser diode provides various advantageous features such ashigh-efficiency of laser oscillation, excellent beam property, excellentvertical mode property, and the like. Further, the vertical-cavity laserdiodes are suitable for constructing a two dimensional array, and thus,there are possibility of application to the art of opticalinterconnection or optical array for laser beam printers.

[0061] In view of the limited length of optical cavity, avertical-cavity laser diode requires to provide a large reflectance.Because of this reason, a distributed Bragg reflector (DBR) is generallyused as the mirror of the vertical optical cavity. By using a DBR, it ispossible to achieve a near 100% reflectance. A DBR is formed by stackingtwo semiconductor layers or dielectric layers having mutually differentrefractive index alternately and repeatedly with an optical distancecorresponding to a quarter of the oscillation wavelength.

[0062] When the difference of refractive index between the twosemiconductor layers constituting a DBR is large, a high reflectance isachieved with a reduced number of repetition. In order to avoid opticalabsorption and to increase the efficiency of laser oscillation, thesemiconductor layers constituting the DBR are required to be transparentto the laser oscillation wavelength.

[0063] In the case of a vertical-cavity laser diode using the materialof an AlGaInP system and oscillating at the wavelength of 630-650 nm, anactive layer of GaInP is formed on a GaAs substrate, and a DBR is formedof high refractive layers of AlGaInP and low refractive layers of AlInP.

[0064] In view of the tendency of increase of bandgap and decrease ofrefractive index with increasing Al content in a semiconductor layercontaining Al, it is desirable to construct a DBR by stacking AlInPlayers and GaInP layers. Unfortunately, a GaInP layer is not transparentto the optical radiation in the wavelength range of 630-650 nm. Thus,there occurs a problem of optical absorption and degradation of opticalcavity efficiency.

[0065]FIG. 8 shows the relationship between the lattice constant andbandgap for the GaInP and AlInP mixed crystals, wherein FIG. 8 shows theΓ valley energy and the X valley energy of the conduction band andfurther the band edge energy of the valence band. As can be seen fromFIG. 8, the bandgap energy increases with decreasing lattice constant inthe foregoing material system.

[0066] In the invention disclosed in the Japanese Laid-Open PatentPublication 9-199793, a DBR is constructed by combining an AlInP/GaInPlayered structure formed on a GaAs substrate with a lattice constantsmaller than the lattice constant of the substrate and an AlGaAs/GaAslayered structure, for reducing the optical loss caused by the DBR.According to the foregoing prior art, a first DBR structure of theAlGaAs/GaAs layered structure is formed on the GaAs semiconductorsubstrate and a second DBR structure of the GaInP/AlInP is formedthereon, with a graded layer interposed between the first and second DBRstructures for relaxing the lattice misfit. On the DBR thus formed, afirst graded cladding layer, a GaInP active layer and a second gradedcladding layer are formed such that the composition grading is symmetricbetween the first and second graded cladding layers. Further, a furtherDBR structure is formed on the second cladding layer.

[0067] The invention disclosed in the foregoing Japanese Laid-OpenPatent Publication 9-199793 is designed so as to minimize the opticalabsorption in the visible wavelength region and to improve the opticalcavity efficiency. The two different material systems are used forconstructing a DBR to eliminate the problem of lattice misfit of theAlGaInP mixed crystal and for avoiding the difficulty of growing a highquality AlGaInP mixed crystal layer. The difficulty of growing anAlGaInP layer will be explained later. Thus, the foregoing prior artuses the material system of AlGaInP for the DBR structure in thevicinity of the active layer where the intensity of optical radiation islarge and uses the material system of AlGaAs for the DBR structure inthe part away from the active layer in order to avoid the problem ofdegradation of the crystal quality associated with the increase of thenumber of stacks.

[0068] Further, there is another prior art vertical-cavity laser diodedisclosed in the Japanese Laid-Open Patent Publication 10-200202 whereinthe vertical-cavity laser diode of this prior art is constructed on aGaInP substrate.

[0069] According to this prior art, a substrate of GaInP having acomposition of Ga_(0.75)In_(0.25)P is used and a DBR of the AlInP/GaInPis formed thereon with lattice matching. On the DBR thus formed, anactive layer of GaInP is formed. According to this prior art, theproblem of degradation of the crystal quality associated with latticemisfit is improved.

[0070] In the case of the forgoing prior art device of the JapaneseLaid-Open Patent Publication 9-199793, it should be noted a plurality ofDBR structures having different lattice constants are provided in asingle laser diode device for changing the lattice constant. Further, inview of the fact that the DBR structure that causes a lattice misfitwith the substrate has a large thickness, the use of the lattice misfitrelaxation layer is not effective for improving the crystal quality. Itshould be noted that the DBR structure that causes a lattice misfit withthe substrate contains at least 20 pairs of layers (40 layers or more)therein.

[0071] In the case of the laser diode disclosed in the JapaneseLaid-Open Patent Publication 200202, a lattice matching is successfullyachieved with respect to the GaInP layer transparent to the opticalradiation in the wavelength range of 635-650 nm by choosing the latticeconstant of the substrate to be smaller than the lattice constant ofGaAs. On the other hand, the laser diode of the foregoing prior art hasa drawback in that increase of Al or Ga content in the AlInP or GaInPmaterial system facilitates hillock formation. Particularly, increase ofAl content causes an extensive hillock formation and causes a seriousproblem in the AlInP material. There is no fundamental solution to thisproblem of hillock formation. When such defects are formed, thehomogeneity of the heteroepitaxial interface is degraded substantially,and the optical scattering associated with such a poor quality interfaceincreases the optical loss. Thereby, the optical cavity efficiency isdeteriorated.

[0072] Further, the invention disclosed in the foregoing JapaneseLaid-Open Patent Publication 10-200202 has a drawback, associated withthe use of the GaInP active layer, in that there is a limitation imposedover the lattice constant when the laser diode is to be operated in thewavelength range of 630-650 nm.

[0073] More specifically, the wavelength of the GaInP mixed crystal thatachieves lattice matching with GaAs is about 650 nm, and the wavelengthbecomes shorter when a GaInP mixed crystal having a lattice constantsmaller than that of GaAs is used for the active layer. In order toachieve the foregoing desired wavelength range, it is thereforenecessary to reduce the Ga content so as to increase the oscillationwavelength of the laser diode. However, such a decrease of the Gacontent causes a compressive strain in the active layer and the qualityof the crystal of the active layer is deteriorated. Thus, the latticeconstant of the active layer is practically limited to the range closeto the lattice constant of GaAs and the degree of freedom in designingthe laser oscillation wavelength is limited.

[0074] On the other hand, the foregoing construction of the JapaneseLaid-Open Patent Publication provides a possibility of increasing thedegree of freedom in the laser diode design associated with thedeviation of lattice constant from the lattice constant of GaAs, such asincreased degree of freedom in selecting the material for various partsof the laser diode. It should be noted that the laser diode of theforegoing Japanese Laid-Open Patent Publication 10-200202 merely focuseson the problem of the optical absorption of the DBR, and no furtherproposals are made with regard to the improvement of other aspects ofthe laser diode.

[0075] There are further rooms for improvement in the vertical-cavitylaser diode having a lattice constant between GaAs and GaP.

[0076] Thus, the present invention provides a vertical-cavity laserdiode operable in the wavelength range of 630-660 nm and various opticalsystems using such a vertical-cavity laser diode.

SUMMARY OF THE INVENTION

[0077] Accordingly, it is a general object of the present invention toprovide a novel and useful laser diode, a vertical-cavity laser diodeand an optical semiconductor device wherein the foregoing problems areeliminated.

[0078] Another and more specific object of the present invention is toprovide a red-wavelength laser diode having a reduced optical waveguideloss and simultaneously a reduced optical damage at an edge surface ofan optical cavity formed in the laser diode.

[0079] Another object of the present invention is to provide a laserdiode, comprising:

[0080] a substrate of a first conductivity type, said substrate having alattice constant of GaAs or a lattice constant between GaAs and GaP;

[0081] a first cladding layer of AlGaInP having said first conductivitytype formed over said substrate;

[0082] an active layer of GaInAsP formed over said first cladding layer;

[0083] an etching stopper layer of GaInP formed over said active layer;

[0084] a pair of current-blocking regions of AlGaInP formed over saidetching stopper layer, said pair of current-blocking regions definingtherebetween a strip region;

[0085] an optical waveguide layer of AlGaInP formed over said pair ofcurrent-blocking regions so as to include said stripe regions, saidoptical waveguide layer covering said etching stopper layer in saidstripe region; and

[0086] a second cladding layer of AlGaInP of a second conductivity typeformed over said optical waveguide layer;

[0087] said current-blocking regions having an Al content substantiallyidentical with an Al content of said second cladding layer.

[0088] According to the present invention, the real refractive-indexincreases in correspondence to the strip region where the opticalwaveguide layer of AlGaInP is formed, and the laser diode has a realrefractive-index waveguide structure characterized by a low opticalloss. Due to the fact that the current-blocking regions outside thestripe region are formed of AlGaInP characterized by a large bandgap,the optical loss caused by such current-blocking regions is successfullyminimized. In view of the fact that the AlGaInP current-blocking regionscontain Al with a concentration substantially identical with the secondcladding layer, which is also formed of AlGaInP, there is no increase ofAl content in these parts of the laser diode. Thereby, the problem ofdamaging at the edge surface of the laser optical cavity caused bynon-optical recombination of carriers, is successfully minimized.

[0089] Another object of the present invention is to provide a laserdiode, comprising:

[0090] a substrate having a lattice constant between GaAs and GaP, saidsubstrate having a first conductivity type;

[0091] a first cladding layer of AlGaInP having said first conductivitytype formed over said substrate;

[0092] a lower optical waveguide layer of GaInP formed over said firstcladding layer;

[0093] an active layer of GaInAsP formed over said lower opticalwaveguide layer;

[0094] a first upper optical waveguide layer of GaInP formed over saidactive layer;

[0095] a pair of current-blocking regions of AlGaInP formed over saidfirst upper optical waveguide layer, said pair of current-blockingregions defining therebetween a stripe region;

[0096] a second upper optical waveguide layer of GaInP formed over saidpair of current-blocking regions so as to include said stripe regions,said second upper optical waveguide layer covering said first upperoptical waveguide layer in said stripe region; and

[0097] a second cladding layer of AlGaInP having a second conductivitytype formed over said second upper optical waveguide layer;

[0098] said current-blocking regions having an Al content generallyidentical with an Al content of said second cladding layer.

[0099] According to the present invention, the laser diode has an SCHstructure in which the active layer is sandwiched vertically by thelower optical waveguide layer and the first upper optical waveguidelayer both free from Al. Thereby, the problem of optical damaging at theedge surface of the laser optical cavity is successfully avoided.

[0100] Another object of the present invention is to provide a laserdiode, comprising:

[0101] a substrate having a lattice constant between GaAs and GaP, saidsubstrate having a first conductivity type;

[0102] a first cladding layer of AlGaInP having said first conductivitytype formed over said substrate;

[0103] a lower optical waveguide layer of GaInP formed over said firstcladding layer;

[0104] an active layer of GaInAsP formed over said lower opticalwaveguide layer;

[0105] a first upper optical waveguide layer formed over said activelayer;

[0106] a pair of current-blocking regions of AlGaInP formed over saidfirst upper optical waveguide layer, said pair of current-blockingregions defining therebetween a stripe region;

[0107] a second upper optical waveguide layer of GaInP formed over saidpair of current-blocking regions so as to include said stripe regions,said second upper optical waveguide layer covering said first upperoptical waveguide layer in said stripe region; and

[0108] a second cladding layer of AlGaInP having a second conductivitytype formed over said second upper optical waveguide layer;

[0109] said current-blocking regions having an Al content generallyidentical with an Al content of said second cladding layer,

[0110] said first upper optical waveguide layer of GaInP and said secondupper optical waveguide layer of GaInP having respective thicknessessuch that a sum of said thickness of said first upper optical waveguidelayer and said thickness of said second upper optical waveguide layer isequal to a thickness of said lower optical waveguide layer of GaInP.

[0111] According to the present invention, the vertical distributionprofile of refractive-index becomes substantially symmetric about theactive layer due the fact that the first and second upper opticalwaveguide layers of GaInP have the total thickness generally identicalwith the thickness of the lower optical waveguide layer of GaInP.Thereby, the optical radiation produced by the laser diode iseffectively confined at the central part of the laser structure and thethreshold of laser oscillation can be reduced.

[0112] Another object of the present invention is to provide a laserdiode operable in the wavelength range of 600-660 nm wherein the deviceperformance is improved by improving the quality of the crystalconstituting the current-blocking regions.

[0113] Another object of the present invention is to provide a laserdiode, comprising:

[0114] a substrate having a first conductivity type;

[0115] a first cladding layer of said first conductivity type providedover said substrate, said first cladding layer having a lattice constantbetween GaAs and GaP;

[0116] an active layer formed over said first cladding layer;

[0117] a second cladding layer of a second conductivity type providedover said active layer, said second cladding layer having said latticeconstant;

[0118] a ridge-stripe region formed in said second cladding layer; and

[0119] a pair of current-blocking regions of said first conductivitytype respectively provided over said second cladding layer at bothlateral sides of said ridge-stripe region;

[0120] each of said current-blocking regions having a compositionrepresented as (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1,0≦y₁≦1, 0.01≦z₁≦1).

[0121] Another object of the present invention is to provide a laserdiode, comprising:

[0122] a substrate having a first conductivity type;

[0123] a first cladding layer of said first conductivity type providedover said substrate, said first cladding layer having a lattice constantbetween GaAs and GaP;

[0124] an active layer formed over said first cladding layer;

[0125] a second cladding layer of a second conductivity type providedover said active layer, said second cladding layer having said latticeconstant;

[0126] a current-blocking layer of said first conductivity typerespectively provided over said second cladding layer;

[0127] a stripe depression formed in said current-blocking layer; and

[0128] a third cladding layer of said second conductivity type formedover said current-blocking layer so as to include said stripedepression,

[0129] said current-blocking layer having a composition represented as(Al_(x2)Ga_(1-x2))_(y2)In_(1-y2)As_(z2)P_(1-z2) (0≦x₂≦1, 0≦y₂≦1,0.01≦z₂≦1).

[0130] According to the present invention, the hillock formation issuccessfully suppressed by incorporating As into said current-blockingregions or current-blocking layer.

[0131] It should be noted that the inventor of the present invention hasdiscovered that the hillock formation is successfully suppressed byincorporating As when growing an AlGaInP mixed crystal layer. It is alsodiscovered that the hillock formation can be reduced by suitablychoosing the condition of growth of the AlGaInP mixed crystal layer suchas increasing growth temperature from 700° C. to 750° C. However, theoptimization of the growth condition was not sufficient for decreasingthe hillock formation to the desired level of hillock density. By addingAs, on the other hand, a remarkable decrease was observed for thehillock density, even in such a case the growth is conducted at thetemperature of 700° C. It is believed that As atoms thus incorporatedsuccessfully suppressed the droplet formation of Al or Ga during theprocess of growing the AlGaInP layer.

[0132] It was further observed that the suppression of hillock formationby As is effective even in such a case in which the amount of the Asatoms incorporated is very small. Naturally, the effect of suppressinghillock formation increases with increasing amount of As in the AlGaInPlayer.

[0133] It should be noted that the foregoing suppression of hillockformation during the growth process of an AlGaInP mixed crystal layer byway of incorporating As is particularly effective when a substratehaving a small offset angle, such as a commercially available GaAsPsubstrate, is used.

[0134] Thus, the laser diode of the present invention has an improvedreliability and lifetime as a result of use of an AlGaInAsP mixedcrystal containing As for the current-blocking regions or for thecurrent-blocking layer. By using the AlGaInAsP mixed crystal for thecurrent-blocking regions or the, current-blocking layer, the flatnessand crystal quality of the device surface are improved. Further, the useof the AlGaInAsP mixed crystal is effective for reducing the leakagecurrent path which is formed inside the laser diode as a result of thehillock formation. Further, the decrease of the hillock density reducesthe optical scattering in the current-blocking regions and the waveguideloss of the laser diode is reduced accordingly. Thereby, the thresholdcurrent of laser oscillation is reduced.

[0135] By using a material transparent to the laser beam produced by thelaser diode, in other words by using a material having a bandgap largerthan a bandgap of the active layer, for the current-blocking regions orlayer, the optical absorption outside the current path region of thelaser diode is reduced. Thereby, the threshold current of laseroscillation is reduced and the efficiency of laser oscillation isimproved. Further, in view of the fact that the AlGaInAsPcurrent-blocking regions or layer, containing a large amount of Al,forms a real refractive-index waveguide structure with the secondcladding layer. It should be noted that the current-blocking regionshave a smaller refractive-index as compared with the second claddinglayer. Thereby, the optical radiation is effectively confined in thestripe region of the laser diode, and the lateral mode of laseroscillation is stabilized. Associated with this, the astigmatism of thelaser diode is reduced.

[0136] Further, by providing a GaInAsP layer in the second claddinglayer or on the current-blocking layer of AlGaInAsP, the GaInAsP layerfunctions as an etching stopper layer with respect to the etchingprocess applied to the second cladding layer or the current-blockinglayer, and the process of forming the stripe ridge structure in thesecond cladding layer or the process of forming the stripe groovestructure in the current-blocking layer as a result of a wet etchingprocess, is facilitated substantially. As a result of use of the GaInAsPetching stopper layer, the height of the stripe ridge structure or thedepth of the stripe groove structure is controlled exactly. Further, theuse of the GaInAsP etching stopper layer protects the surface of thesecond cladding layer or the current-blocking layer from being exposedto the air after the etching process, and the problem of surfaceoxidation of Al in the second cladding layer or in the current-blockinglayer is successfully avoided. It should be noted that a GaAsPcomposition acts as an effective etching stopper against an etchingprocess applied to an AlGaInAsP layer by a hydrochloric acid etchant,while a GaInP composition acts as an effective etching stopper againstan etching process applied to an AlGaInAsP layer by a phosphoric orsulfuric acid etchant.

[0137] Further, the use of the optical waveguide layer of GaInP adjacentto the active layer eliminates the direct contact of the active layerand the cladding layer that contains Al, and the problem of damaging ofthe laser cavity edge surface caused by Al is effectively eliminated.Thereby, it becomes possible to operate the laser diode with a highoutput power.

[0138] Another object of the present invention is to provide asemiconductor light-emitting device formed of a semiconductor materialhaving a lattice constant between GaP and GaAs wherein the fabricationprocess is simplified.

[0139] Another object of the present invention is to provide asemiconductor light-emitting device formed of a semiconductor materialhaving a lattice constant between GaP and GaAs wherein the opticalwaveguide loss is minimized and suitable for increasing output opticalpower.

[0140] Another object of the present invention is to provide asemiconductor light-emitting device, comprising:

[0141] a semiconductor substrate;

[0142] an active layer provided over said semiconductor substrate, saidactive layer emitting optical radiation;

[0143] a semiconductor layer vertically sandwiching said active layerwith another semiconductor layer, said semiconductor layer having abandgap larger than a bandgap of said active layer and a latticeconstant between GaP and GaAs, said semiconductor layer containing a tobe-oxidized layer in a part thereof with a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t)(0.8≦x≦1, 0≦y≦0.2, 0≦t≦1),

[0144] a part of said to be-oxidized layer being oxidized to form aselective oxidation region.

[0145] According to the present invention, the selective oxidationregion forms a current-blocking structure for confining an injectedelectric current. In view of the fact that the selective oxidationregion thus formed has a reduced refractive index, there appears adifference in the real refractive index between the part of theAlGaInPAs to-be-oxidized layer where the oxidized region is formed andthe current path region where no such oxidized region is formed. Inother words, the current-blocking structure thus formed by the selectiveoxidation of the AlGaInPAs layer functions also as the real refractiveindex waveguide structure effective for lateral mode control. As thereal refractive index waveguide structure thus formed contains, in thevicinity of the active layer, only the material which is free fromwaveguide less for all the wavelengths, the laser diode is easilyoperated to produce a large output optical power.

[0146] It should be noted that the foregoing advantageous structure canbe formed by a single crystal growth process. Thereby, the semiconductorlight-emitting device of the present invention can be fabricated easilywith high yield.

[0147] Another object of the present invention is to provide asemiconductor light-emitting device, comprising:

[0148] a semiconductor substrate;

[0149] an active layer provided over said semiconductor substrate, saidactive layer producing optical radiation; and

[0150] a pair of cladding layers sandwiching said active layervertically,

[0151] said active layer being one of a single quantum well structurecontaining therein a quantum well layer and a multiple quantum wellstructure containing therein a quantum well layer and a barrier layer,

[0152] said quantum well layer comprising a mixed crystal of AlGaInPAshaving a composition represented as(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1,0≦t₁≦1),

[0153] said barrier layer comprising a mixed crystal of AlGaInPAs havinga composition represented as(Al_(x2)Ga_(1-x2))_(α2)In_(1-α2)P_(t2)As_(1-t2) (0≦x₂<1, 0.5<α₂<1,0≦t₂≦1),

[0154] each of said cladding layers comprising a mixed crystal ofAlGaInPAs containing Al and having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1), eachof said cladding layers having a lattice constant between GaP and GaAsand a bandgap larger than a bandgap of said active layer,

[0155] an optical waveguide layer of AlGaInPAs interposed between saidactive layer and each of said cladding layers, said optical waveguidelayer having a bandgap larger than the bandgap of said active layer butsmaller than the bandgap of said cladding layer, said optical waveguidelayer having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ<1, 0<u≦1),

[0156] a to-be-oxidized layer provided in at least one of said claddinglayers such that said cladding layer contains said to-be-oxidized layerin correspondence to a part thereof, or between said active layer andone of said cladding layers, said to-be-oxidized layer having acomposition represented as Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1,0≦y≦0.2, 0≦t≦1), a part of said to-be-oxidized layer being selectivelyoxidized to form a selective oxidized region.

[0157] According to the present invention, it is possible to oscillatethe laser diode in the visible wavelength band in view of the fact thatthe active layer is formed of a mixed crystal of AlGaInPAs having acomposition represented as(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1,0≦t₁≦1). In view of the fact that a mixed crystal of AlGaInPAscontaining Al with a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1) andhaving a lattice constant between GaP and GaAs is used for the claddinglayer, the bandgap of the cladding layer is increased as compared withthe case of using a cladding layer having a lattice matching compositionto the GaAs substrate, and the wavelength of the output opticalradiation of the semiconductor light-emitting device is reduced.

[0158] Further, in view of the fact that the semiconductorlight-emitting device of the present invention employs the an SCHstructure in which the mixed crystal of(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1, 0≦t₁≦1)is used for the quantum well layer forming the active layer and in whichthe mixed crystal of (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1,0.5<γ<1, 0<u≦1) is used for the optical waveguide layer, a wide bandgapcan be realized with a reduced Al content as compared with the case ofusing a material forming a lattice matching with a GaAs substrate, andthe non-optical recombination of carriers is reduced substantially.Associated with this, the efficiency of optical emission is improved. Inthe case of a laser diode, the problem of damaging of optical cavityedge surface as a result of the non-optical recombination of carries isreduced and the laser diode can be operated stably and reliably withhigh optical output power.

[0159] Further, it is possible to induce a strain in the semiconductorlight-emitting device of the present invention with respect to thecladding layer. In this case, bandgap of the active layer can bereduced. Further, a large conduction band discontinuity can be realizedin the semiconductor light-emitting device of the present invention byreducing the Al content in the optical waveguide layer. Thereby, theproblem, pertinent to a conventional red-wavelength laser diode of theAlGaInP system, of carrier overflow taking place on the conduction band,is reduced substantially.

[0160] By interposing the to-be-oxidized layer in a part of one or bothof the two cladding layers or at the interface between the active layerand one of the cladding layers with the composition ofAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), followedby an oxidizing process oxidizing a part of the to-be-oxidized layer, itis possible to form a current-blocking region by the oxidized regionthus formed selectively in the to-be-oxidized layer. As the oxidizedregion thus formed has a reduced refractive index, there is also formeda real refractive index waveguide structure by the part of theto-be-oxidized region where the selective oxidation has occurred and bythe part where no such a selective oxidation has occurred. Thereby thelateral mode control becomes possible in the semiconductorlight-emitting device. The real refractive index waveguide structurethus formed contains, in the vicinity of the active layer, only thematerial which is free from waveguide less for all the wavelengths, thelaser diode is easily operated to produce a large output optical power.

[0161] It should be noted that the foregoing advantageous structure canbe formed by a single crystal growth process. Thereby, the semiconductorlight-emitting device of the present invention can be fabricated easilywith high yield.

[0162] Another object of the present invention is to provide asemiconductor light-emitting device, comprising:

[0163] a GaAs substrate;

[0164] an active layer provided over said GaAs substrate, said activelayer emitting an optical radiation;

[0165] a pair of semiconductor layers sandwiching said active layervertically,

[0166] said semiconductor layer containing a to be-oxidized layer in apart thereof with a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) andcontaining P,

[0167] a part of said to be-oxidized layer being oxidized to form aselective oxidation region.

[0168] According to the present invention, it is possible to achieve alattice matching for the to-be-oxidized layer with respect to the GaAssubstrate by introducing P therein. Thereby, the adversary effect ofstrain caused in the to-be-oxidized layer is effectively eliminated.

[0169] Another object of the present invention is to provide asemiconductor light-emitting device, comprising:

[0170] a GaAs substrate;

[0171] an active layer of an AlGaInP system formed over said GaAssubstrate, said active layer emitting optical radiation;

[0172] a pair of semiconductor layers sandwiching said active layervertically, each of said semiconductor layers having a bandgap largerthan a bandgap of said active layer,

[0173] said semiconductor layers including, in a part thereof, a layerof AlGaInAs having a composition represented as Al_(x)Ga_(y)In_(1-x-y)As(0.8≦x≦1, 0≦y≦0.2),

[0174] a part of said semiconductor layer being oxidized to form a pairof oxidized regions, with a not-oxidized region formed therebetween witha width w1, a total width of said pair of oxidized regions being definedas w2,

[0175] wherein said width w1 is set such that a ratio of said width w1with respect to a sum of said width w1 and said width w2, defined asw1/(w1+w2) is smaller than 0.6.

[0176] According to the present invention, the waveguide loss caused bythe variation of the edge width is successfully eliminated by settingthe foregoing width to be smaller than 0.6.

[0177] Another object of the present invention is to provide asemiconductor light-emitting device, comprising:

[0178] a GaAs substrate;

[0179] an active layer provided over said GaAs substrate, said activelayer emitting optical radiation; and

[0180] a pair of semiconductor layers sandwiching said active layervertically, each of said semiconductor layers having a bandgap largerthan a bandgap of said active layer,

[0181] said semiconductor layers including, in a part thereof, a layerof AlGaInAs having a composition represented as Al_(x)Ga_(y)In_(1-x-y)As(0.8≦x≦1, 0≦y≦0.2),

[0182] a part of said semiconductor layer being oxidized to form anoxidized region,

[0183] a ridge structure being formed in a part of said semiconductorlayer located at least above said layer of AlGaInAs, said ridgestructure having a ridge width exceeding 10 μm.

[0184] According to the present invention, a large contact area forelectrode is secured by setting the ridge width to be larger than 10 μmand the differential resistance during the device operation is reduced.Further, the structure is suitable for a flip-chip mounting in which theheat of the active region is efficiently dissipated to a supportingsubstrate via the electrode.

[0185] Another object of the present invention is to provide avertical-cavity laser diode operable in the wavelength range of 630-660nm and various optical systems using such a vertical-cavity laser diode.

[0186] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0187] a substrate;

[0188] an active layer provided over said substrate, said active layeremitting optical radiation; and

[0189] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector comprising a plurality of layers stackedover said substrate,

[0190] said distributed Bragg reflector having a lattice constantbetween GaAs and GaP and including at least two semiconductor layers ofrespective, mutually different compositions,

[0191] at least one of said semiconductor layers having a compositionrepresented as (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1) As_(z1)P_(1-z1) (0≦x₁≦1,0.5≦y₁≦1, 0<z₁>1).

[0192] According to the present invention, the distributed Braggreflector (DBR) is formed of an AlInAsP mixed crystal containing thereinAs. Thereby, the hillock formation on the surface of the layersconstituting the DBR is substantially completely suppressed. Thereby,the problem of optical loss associated with the hillocks formed in theDBR is eliminated and the reflectance of the DBR is improved remarkably.With the improvement in the reflectance of the DBR, the oscillationthreshold of the laser diode is improved and the device performance anddevice lifetime are improved also. The improvement becomes appreciablewhen As is added with a concentration of about 1%.

[0193] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0194] a substrate;

[0195] an active layer provided over said substrate, said active layeremitting optical radiation; and

[0196] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector comprising a plurality of layers stackedover said substrate,

[0197] said active layer having a composition represented asGa_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1).

[0198] According to the present invention, the desired laser oscillationwavelength is realized with reduced strain as a result of use of GaInAsPfor the active layer. Thereby, the surface morphology of the activelayer is improved, and the efficiency of laser oscillation is improvedas a result of the improvement of quality of crystal of the activelayer. Further, as a result of reduced strain in the active layer, thedegree of freedom for designing the laser diode is improved. As theactive layer has a lattice constant closer to GaP or AlP as comparedwith the prior art vertical-cavity laser diode, it becomes possible touse a layer of AlInAsP for the DBR. As the layer of AlInAsP has a smallrefractive index, the number of stacks of the layers in the DBR isreduced, and the resistance of the laser diode is accordingly reduced.

[0199] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0200] a substrate;

[0201] an active layer provided over said substrate, said active layeremitting optical radiation;

[0202] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP; and

[0203] a pair of semiconductor layers having a composition representedas Ga_(y3)In_(1-y3)P (0.5≦y₃≦1) provided at upper and lower sides ofsaid active layer.

[0204] According to the present invention, it is possible to reduce thenumber of non-optical recombination centers associated with Al byproviding the GaInP layers at both upper and lower sides of the activelayer. Further, the problem of multiplication of crystal defectsoriginating from Al, or the problem of migration of the crystal defectsinto the active region of the laser diode, is also reduced and thereliability of the laser diode is improved.

[0205] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0206] a substrate;

[0207] an active layer provided over said substrate, said active layeremitting optical radiation;

[0208] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP;

[0209] a contact layer provided over said distributed Bragg reflector;and

[0210] an electrode provided on said contact layer in ohmic contacttherewith,

[0211] said contact layer being transparent to an optical beam producedas a result of interaction of said optical radiation produced by saidactive layer with said distributed Bragg reflector.

[0212] According to the present invention, the process of eliminating apart of the contact layer in correspondence to an optical window, fromwhich the optical beam is emitted to the region outside the laser diode,is eliminated as a result of use of a material transparent to theoptical beam for the contact layer.

[0213] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0214] a substrate;

[0215] an active layer provided over said substrate, said active layeremitting optical radiation; and

[0216] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP,

[0217] said distributed Bragg reflector including therein asemiconductor layer having a composition represented asAlAs_(z4)P_(1-z4) (0≦z₄≦1).

[0218] According to the present invention that uses AlAsP characterizedby a small refractive index as compared with AlInP of the same latticeconstant, it becomes possible to increase the diffraction indexdifferent or step inside the DBR and the number of stacks of layers inthe DBR can be reduced. Associated therewith, the threshold current oflaser oscillation is reduced together with the device resistance.

[0219] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0220] a substrate;

[0221] an active layer provided over said substrate, said active layeremitting optical radiation; and

[0222] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted from said active layer ina direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP,

[0223] said distributed Bragg reflector including therein asemiconductor layer having a composition represented asAlAs_(z5)P_(1-z5) (0≦z₅≦1),

[0224] said semiconductor layer being laterally sandwiched by a pair ofoxide regions formed coplanar to said semiconductor layer, saidsemiconductor layer and said pair of oxide regions forming a currentconfinement structure.

[0225] According to the present invention, the oxidized regions areformed in the form of high-quality insulator by selective oxidationprocess of a semiconductor layer containing Al. Larger the Al content,easier for the selective oxidation process. Particularly, an AlAsP mixedcrystal, which contains Al as the sole group III element, is easy foroxidation. According to the present invention, the threshold current oflaser oscillation is decreased as a result of formation of the currentconfinement structure.

[0226] Another object of the present invention is to provide avertical-cavity laser diode, comprising:

[0227] a substrate;

[0228] an active layer provided over said substrate, said active layeremitting optical radiation;

[0229] a distributed Bragg reflector provided over said substrate in anoptical path of said optical radiation emitted perpendicularly to aplane of said active layer, said distributed Bragg reflector having alattice constant between GaAs and GaP; and

[0230] a semiconductor layer interposed between said active layer andsaid distributed Bragg reflector, said semiconductor layer having acomposition represented as AlAs_(z6)P_(1-z6) (0≦z₆≦1),

[0231] said semiconductor layer being laterally sandwiched by a pair ofoxidized regions formed coplanar to said semiconductor layer.

[0232] According to the present invention, a current confinementstructure is formed between the DBR and the active layer by applying aselective oxidation process to the semiconductor layer. As the currentconfinement structure is thus formed in the vicinity of the activelayer, the current is injected to the active layer in the form of highlyconfined state, and lateral spreading of the carriers in the activelayer is effectively suppressed. Further, the refractive indexdistribution in the layer containing the semiconductor layer and theoxidized regions enables an effective control of lateral mode of laseroscillation. Thus, the laser diode of the present invention oscillatesat low threshold current with a stabilized lateral mode.

[0233] Other objects and further features of the present invention willbecome apparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0234]FIG. 1 is a diagram showing the construction of a conventionallaser diode of edge-emission type;

[0235]FIG. 2 is a diagram showing the construction of anotherconventional laser diode of edge-emission type;

[0236]FIG. 3 is a diagram showing the construction of a furtherconventional laser diode of edge-emission type;

[0237]FIG. 4 is a diagram showing the construction of a furtherconventional laser diode of edge-emission type;

[0238]FIG. 5 is a diagram showing the relationship between bandgapenergy and lattice constant for a III-V semiconductor material system;

[0239]FIG. 6 is another diagram showing the relationship between bandgapenergy and lattice constant for a III-V semiconductor material system;

[0240]FIG. 7 is a diagram showing the construction of a conventionallaser diode of edge-emission type;

[0241]FIG. 8 is another diagram showing the relationship between bandgapenergy and lattice constant for a III-V semiconductor material system;

[0242]FIG. 9 is a diagram showing the construction of a laser diodeaccording to a first embodiment of the present invention;

[0243] FIGS. 10A-10D are diagrams showing the fabrication process of thelaser diode of FIG. 9;

[0244]FIG. 11 is a diagram showing the construction of a laser diodeaccording to a second embodiment of the present invention;

[0245]FIG. 12 is a diagram showing the construction of a laser diodeaccording to a third embodiment of the present invention;

[0246]FIG. 13 is a diagram showing the construction of a laser diodeaccording to a fourth embodiment of the present invention;

[0247]FIG. 14 is a diagram showing the construction of a laser diodeaccording to a fifth embodiment of the present invention;

[0248]FIG. 15 is a diagram showing the construction of a laser diodeaccording to a sixth embodiment of the present invention;

[0249]FIG. 16 is a diagram showing the construction of a laser diodeaccording to a seventh embodiment of the present invention;

[0250]FIG. 17 is a diagram showing the construction of a laser diodeaccording to an eighth embodiment of the present invention;

[0251]FIG. 18 is a diagram showing the construction of a laser diodeaccording to a ninth embodiment of the present invention;

[0252]FIG. 19 is a diagram showing the construction of a laser diodeaccording to a tenth embodiment of the present invention;

[0253]FIG. 20 is a diagram showing the construction of a laser diodeaccording to an eleventh embodiment of the present invention;

[0254]FIG. 21 is a diagram showing the construction of a laser diodeaccording to a twelfth embodiment of the present invention;

[0255]FIG. 22 is a diagram showing the construction of a laser diodeaccording to a thirteenth embodiment of the present invention;

[0256]FIG. 23 is a diagram showing the construction of a laser diodeaccording to a fourteenth embodiment of the present invention;

[0257]FIG. 24 is a diagram showing the construction of a laser diodeaccording to a fifteenth embodiment of the present invention;

[0258]FIG. 25 is a diagram showing the construction of a laser diodeaccording to a sixteenth embodiment of the present invention;

[0259]FIG. 26 is a diagram showing the construction of a laser diodeaccording to a seventeenth embodiment of the present invention;

[0260]FIG. 27 is a diagram showing the construction of a laser diodeaccording to an eighteenth embodiment of the present invention;

[0261]FIG. 28 is a diagram showing the construction of a laser diodeaccording to a nineteenth embodiment of the present invention;

[0262]FIG. 29 is a diagram showing the construction of a laser diodeaccording to a twentieth embodiment of the present invention;

[0263]FIG. 30 is a diagram showing the construction of a laser diodeaccording to a twenty-first embodiment of the present invention;

[0264]FIG. 31 is a diagram showing the construction of a laser diodeaccording to a twenty-second embodiment of the present invention;

[0265]FIG. 32 is a diagram showing the construction of a laser diodeaccording to a twenty-third embodiment of the present invention;

[0266]FIG. 33 is a diagram showing a part of a laser diode according toa twenty-fourth embodiment of the present invention;

[0267]FIG. 34 is a diagram showing a selective oxidation used in thefabrication process of the laser diode of the twenty-fourth embodiment;

[0268]FIG. 35 is a diagram showing the construction of a laser diodeaccording to a twenty-fifth embodiment of the present invention;

[0269]FIG. 36 is a diagram showing the construction of a laser diodeaccording to a twenty-sixth embodiment of the present invention;

[0270]FIG. 37 is a diagram showing the construction of a laser diodeaccording to a twenty-seventh embodiment of the present invention;

[0271]FIG. 38 is a diagram showing the construction of a laser diodeaccording to a twenty-eighth embodiment of the present invention;

[0272]FIG. 39 is a diagram showing the construction of a laser diodeaccording to a twenty-ninth embodiment of the present invention;

[0273]FIG. 40 is a diagram showing the construction of a laser diodeaccording to a thirtieth embodiment of the present invention;

[0274]FIG. 41 is a diagram showing the construction of a laser diodeaccording to a thirty-first embodiment of the present invention;

[0275]FIG. 42 is a diagram showing the construction of a laser diodeaccording to a thirty-second embodiment of the present invention;

[0276]FIG. 43 is a diagram showing the construction of a xerographicimage recording apparatus according to a thirty-third embodiment of thepresent invention;

[0277]FIG. 44 is a diagram showing the construction of an optical diskdrive according to a thirty-fourth embodiment of the present invention;

[0278]FIG. 45 is a diagram showing the construction of an optical moduleaccording to a thirty-fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0279] [First Embodiment]

[0280]FIG. 1 shows the structure of a laser diode according to a firstembodiment of the present invention.

[0281] Referring to FIG. 1, the laser diode is constructed on a GaAsPsubstrate 115, wherein the GaAsP substrate 115 is formed by stacking, ona GaAs substrate 101 of n-type, a GaAsP composition graded layer 102 ofn-type and a GaAs_(0.6)P_(0.4) thick film 103 of p-type.

[0282] On the GaAsP substrate 115, there is provided a cladding layer104 of n-type AlGaInP having a composition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)P, and an active layer 105 of GaInAsP isformed on the cladding layer 104. Further, an etching stopper layer 106of GaInP is formed on the active layer with the composition ofGa_(0.7)In_(0.3)P.

[0283] On the etching stopper layer 106, there are formed a pair ofcurrent-blocking regions of p-type AlGaInP 107 at both lateral sides ofa stripe region, in which a current injection is made, wherein thecurrent-blocking regions 107 have a composition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)P. On each of the current-blockingregions 107, there is provided another current-blocking region 108 ofn-type AlGaInP having a composition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)P.

[0284] On the current blocking regions 108 thus formed, there isprovided an optical waveguide layer 109 of AlGaInP having a compositionrepresented as (Al_(0.1)Ga_(0.9))_(0.7)In_(0.3)P, wherein the opticalwaveguide layer 109 covers the stripe region where the etching stopperlayer 106 is exposed.

[0285] On the optical waveguide layer 109, there is provided a claddinglayer 110 of p-type AlGaInP having a composition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)P. Further, a band-discontinuityrelaxation layer 111 of p-type GaInP is formed on the cladding layer 110with the composition of Ga_(0.7)In_(0.3)P. Further, a cap layer 112 ofp-type GaAsP is formed on the band-discontinuity relaxation layer 111with the composition of GaAs_(0.6)P_(0.4).

[0286] Further, a p-type electrode 113 is formed on the surface of thep-type cap layer 112 and an n-type electrode 114 is formed on the bottomsurface of the GaAs substrate 101.

[0287] Next, the fabrication process of the laser diode of FIG. 9 willbe described with reference to FIGS. 10A-10D.

[0288] Referring to FIG. 10A, the cladding layer 104, the active layer105, the etching stopper layer 106, a p-type AlGaInP layer correspondingto the current-blocking regions 107, and an n-type AlGaInP layercorresponding to the current-blocking regions 108 are grown epitaxiallyon the GaAsP substrate 115 by an MOCVD process. The active layer 105 mayhave a composition tuned to the bandgap wavelength of 635 nm.

[0289] Next, a resist film 201 is formed on the AlGaInP layercorresponding to the current-blocking regions 108 and a stripe window isformed in the resist film 201 by applying a photolithographic process.Further, the AlGaInP layers corresponding to the current-blockingregions 108 and the current-blocking regions 107 are patterned by achemical etching process while using the resist film as a mask, untilthe etching stopper layer 106 is exposed in correspondence to the striperegion. As a result, a stripe groove as represented in FIG. 10B isformed, and the current blocking regions 107 are separated from eachother by the central stripe groove. Similarly, the current-blockingregions 108 are separated from each other by the stripe groove. Thechemical etching process may be conducted by using a sulfuric solutionas an etchant.

[0290] Next, the resist film 201 is removed and the optical waveguidelayer 109, the cladding layer 110, the band-discontinuity relaxationlayer 111, and the cap layer 112 are formed consecutively by anepitaxial process. Thereafter, the p-type electrode 113 is formed on thecap layer 112 and the bottom surface of the GaAs substrate 101 ispolished. Finally, the n-type electrode 114 is formed on the polishedbottom surface of the GaAs substrate 101.

[0291] The laser diode of FIG. 9 has s current-confinement structureformed by the current-blocking regions 107 and 108, wherein thecurrent-blocking regions 107 and 108 confine the injected drive currentinto the stripe region thus formed.

[0292] As the current-confinement structure thus formed include astacking of the p-type AlGaInP layer 107 and the n-type AlGaInP layer108, there is formed a pnpn junction in the region outside the striperegion. Because of the reverse biasing of the pn junction, there flowsno substantial electric current in such a current-confinement structure,and the electric current is effectively confined into the stripe region.

[0293] Of course it is possible to construct the current-confinementstructure by stacking of more than two layers with different carrierdensity or different conductivity type. Alternatively, thecurrent-confinement structure may be formed by using a high-resistanceor semi-insulating AlGaInP layer.

[0294] When a drive current is injected into the GaInAsP active layer105, there occurs emission of optical radiation with a wavelength of 635nm in correspondence to the bandgap. Thereby, it should be noted thatthe optical waveguide layer 109 of AlGaInP covers the stripe groove overthe thin GaInP etching stopper layer 106. In view of the fact that theoptical waveguide layer 109 has a refractive index smaller than therefractive index of the active layer 105 but larger than the refractiveindex of the cladding layer 110 or the current-blocking regions 107 and108, and further in view of the fact that the optical waveguide layer109 is located away from the active layer 105 in the region outside thestripe groove, there is formed a refractive index structure in which therefractive index is larger in the stripe groove than in the regionoutside the strip groove. Thereby, the optical radiation emitted by theactive layer 105 is effectively confined in the stripe groove.

[0295] While it is true that the horizontal lateral mode leaks into theregion outside the stripe groove, the optical loss outside the stripegroove is minimized due to the large bandgap of the p-typecurrent-blocking regions 107 and the n-type current-blocking regions108. There occurs no substantial optical absorption. Thus, the drivecurrent of the laser diode is effectively minimized.

[0296] In the structure of FIG. 9, it should be noted that the foregoingreal refractive index profile is formed, not by reducing the refractiveindex of the current-blocking regions 107 and 108 but by changing thelocation of the optical waveguide layer 109. Thus, the same compositioncan be used for the p-type current-blocking regions 107 and the n-typecurrent-blocking regions 108. Thereby, there is no need of increasingthe Al content and the problem of optical damaging of the optical cavityedge is minimized.

[0297] In view of the fact that the laser diode of FIG. 9 is constructedon the GaAsP substrate 115 having a lattice constant between the latticeconstant of GaAs and the lattice constant of GaP, the GaInP etchingstopper layer 106 that achieves lattice matching with the GaAsPsubstrate 115 has the composition of Ga_(0.7)In_(0.3)P and a bandgapwavelength of 560 nm. As this wavelength is substantially shorter thanthe bandgap wavelength of 635 nm of the active layer 105, the etchingstopper layer 106 functions as a carrier-blocking layer having a bandgaplarger than the bandgap of the active layer 105. Thereby, there occursno optical absorption by the etching stopper layer 106. In view of thefact that a GaInP layer shows a very low etching rate with respect to anAlGaInP layer when subjected to an etching process using a sulfuric acidetchant, the selective etching process for forming the current-blockingregions 107 and 108 is substantially facilitated.

[0298] It should be noted that the GaAs_(0.6)P_(0.4) 115 is formed onthe n-type GaAs substrate 101 as a result of stacking of the n-typeGaAsP composition graded layer 102 and the n-type GaAs_(0.6)P_(0.4)thick film 103 formed by a vapor-phase epitaxial process, as notedpreviously. Such a GaAsP substrate is marketed commercially as asubstrate for 660 nm-wavelength red LED. Thus, fabrication of the laserdiode is easily made, by utilizing such a commercially availablesubstrate.

[0299] [Second Embodiment]

[0300] Next, a second embodiment of the present invention will bedescribed with reference to FIG. 11 wherein those parts corresponding tothe parts described previously are designated by the same referencenumerals and the description thereof will be omitted.

[0301] Referring to FIG. 11, there is provided a lower optical waveguidelayer 301 on the n-type cladding layer 104 with a composition ofGa_(0.7)In_(0.3)P, and a quantum-well active layer 302 is formed on thelower optical waveguide layer 301 by alternately stacking GaInP andGaInAsP layers so as to form a strained superlattice structure. Further,a first upper optical waveguide layer 303 is formed on the quantum-wellactive layer 302 with a composition represented as Ga0.7In0.3P. Further,a second upper optical waveguide layer 304 is formed on the n-typecurrent-blocking regions 108 with a composition of Ga0.7In0.3P, whereinthe second upper optical waveguide layer is formed on thecurrent-blocking regions 108 so as to cover the stripe groove region.Otherwise, the laser diode of FIG. 11 has a construction disclosed inFIG. 9.

[0302] In the present embodiment, the laser diode has an SCH structureas a result of use of the optical waveguide layers 301 and 303 on theGaInAsP/GaInP quantum-well active layer 302, wherein it should be notedthat the lower optical waveguide layer 301 and the first upper opticalwaveguide layer 303 have an Al-free composition of Ga0.7In0.3P.

[0303] Thus, the laser diode of the present embodiment has anadvantageous feature over the laser diode of the previous embodiment inthe point in that the problem of oxidation or formation of surfacestates at the optical cavity edge surface is reduced. Thereby, theproblem of optical damaging at such a cavity edge surface is reduced.

[0304] Further, there is an advantageous point,-in view of the fact thatthe second upper optical waveguide layer 304 covering the first opticalwaveguide layer 303 and the n-type current-blocking regions 108 isformed with the composition of Ga0.7In0.3P, that the second crystalgrowth is started from the layer thus free from Al. Thereby, the qualityof the crystal of the semiconductor layers thus formed by the regrowthprocess is improved.

[0305] Further, it should be noted, in the laser diode of FIG. 11, thatthe total thickness of the Ga_(0.7)In_(0.3)P first upper opticalwaveguide layer 303 and the Ga_(0.7)In_(0.3)P second upper opticalwaveguide layer 304 is set generally equal to the thickness of theGa_(0.7)In_(0.3)P lower optical waveguide layer 301. Thus, there appearsa symmetric refractive profile about the quantum-well active layer 302in the vertical cross section of the laser diode for the part thatincludes the stripe region. As a result of such a vertically symmetricrefractive index profile, the quantum-well active layer 302 is locatedat the position where the vertical mode optical intensity is maximum.Thereby, the coefficient of optical confinement is improved and thethreshold current of laser oscillation is reduced.

[0306] [Third Embodiment]

[0307]FIG. 12 shows the construction of a laser diode according to athird embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0308] Referring to FIG. 12, the laser diode includes an n-type claddinglayer 401 formed on the n-type GaAs_(0.6)P_(0.4) thick film 103 with acomposition of (Al_(0.5)Ga₀₋₅)_(0.7)In_(0.3)As_(0.05)P_(0.95). Further,the laser diode includes a pair of current-blocking regions 402 ofp-type AlGaInAsP formed on the first upper optical waveguide layer 303with the composition of(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)As_(0.05)P_(0.95) except for the striperegion, and a pair of current-blocking regions 403 of n-type AlGaInAsPhaving a composition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)As_(0.05)P_(0.95) are formed on thep-type current-blocking regions 402 respectively. Each of the n-typecurrent-blocking regions 403 is covered by a cap layer 404 of GaInPhaving a composition represented as Ga_(0.7)In_(0.3)P.

[0309] Further, the laser diode includes, on the second upper opticalwaveguide layer 304, a cladding layer 405 of p-type AlGaInAsP with acomposition represented as(Al_(0.5)Ga_(0.5))_(0.7)In_(0.3)As_(0.05)P_(0.95).

[0310] In the present embodiment, the n-type AlGaInAsP current-blockingregions 403 are covered with the Ga_(0.7)In_(0.3)P cap layer 404, whichis free from Al. Thereby, the surface of the current-blocking regions403 containing Al is not exposed for the surface on which the regrowthprocess is to be conducted. Thus, the quality of the crystal layers tobe grown thereon is improved.

[0311] Further, it should be noted that the n-type cladding layer 401,the p-type current-blocking regions 402, the n-type current-blockingregions 403, and the p-type cladding layer 405 are formed of AlGaInAsPcontaining As with an amount of about 5%. By adding a small amount of Asto the mixed crystal of AlGaInP, the hillock density and hillock sizeare reduced substantially in the mixed crystal layer grown by an MOCVDprocess. Thereby, the smoothness of the device surface is improved andthe scattering loss of the optical radiation in the optical waveguide isminimized. Associated therewith, the threshold current of laseroscillation is reduced and the slope efficiency is improved.

[0312] According to the present embodiment, it is possible to set awidth W of the stripe region to be smaller than 5 μm. For example, it ispossible to set the width to 3 μm. When the width of the stripe regionis thus decreased, the leakage of the lateral mode optical radiation tothe region outside the stripe region increases inevitably. If thecurrent-blocking regions of the laser diode are formed of a materialthat absorbs the optical radiation, there would occur an extensiveoptical absorption loss and the slope efficiency of the laser diodewould have been deteriorated. Further, when the Al-content in thecurrent-blocking regions is large, there is a risk of optical damagingcaused in the current-blocking regions in correspondence to the opticalcavity edge as a result of optical absorption by the surface states. Itshould be noted that the current-blocking regions contain Al with aconcentration identical with the concentration of the cladding layer,and there occurs no increase of optical damaging in the current-blockingregions. The decrease of the stripe width of course contributes to thedecrease of the drive current of the laser diode.

[0313] [Fourth Embodiment]

[0314]FIG. 13 shows the construction of a laser diode according to afourth embodiment of the present invention.

[0315] Referring to FIG. 13, the laser diode is constructed on asubstrate 1102 of n-type GaAs carrying thereon a composition-gradedlayer 1103 of n-type GaAsP having a composition represented asGaAs_(y)P₁,y, wherein the composition-graded layer 1103 is formed by anMOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1103 is well established a smooth surface is realized byoptimizing the composition gradient.

[0316] Next, a buffer layer 1104 of n-type GaAsP having a composition ofGaAs_(0.6)P₀₄ is grown on the composition-graded layer 1103, andcladding layer 1105 of n-type AlGaInAsP, an undoped active layer 1106 ofGaInAsP, a cladding layer 1107 of p-type AlGaInAsP, a spike-eliminationlayer 1108 of p-type GaInP, and a cap layer 1109 of p-type GaAsP, aregrown consecutively on the buffer layer 1104 by an MOCVD process.

[0317] After the formation of the cap layer 1109, an SiO₂ film isdeposited by a CVD process, followed by a photolithographic process toform a stripe pattern in correspondence to the region where injection ofelectric current is to be made, with a width of 10 μm.

[0318] Next, the layers 1109 and 1108 are patterned consecutively by achemical etching process while using the SiO₂ film thus formed as amask, wherein the chemical etching process is continued until theetching reaches a part of the semiconductor layer 1107. As a result, amesa ridge stripe is formed as represented in FIG. 13.

[0319] In the foregoing chemical etching process, the p-type GaAs caplayer 1109 is patterned while using a sulfuric acid etchant, while thep-type GaInP layer 1108 and the p-type AlGaInAsP layer 1107 arepatterned by a hydrochloric acid etchant. The depth of etching of thecladding layer 1107 is controlled by way of controlling the duration ofthe etching process. According to such a process, it is possible tosimplify the fabrication process and device structure.

[0320] Next, a pair of current-blocking regions 1110 of n-type AlGaInAsPare formed on the mesa structure thus formed by a regrowth process whileusing the SiO₂ film as the mask covering the ridge region of the mesastructure, wherein the current-blocking regions 1110 are grown on theregion of the cladding layer 1107 not covered by the SiO₂ mask.

[0321] Further, the SiO₂ mask is removed and a contact layer 1111 ofp-type GaAsP is grown on the current-blocking regions 1110 by a regrowthprocess so as to cover the p-type GaAsP cap layer 1109 exposed at theridge region of the mesa structure.

[0322] Thereafter, the bottom surface of the substrate 1102 is polishedand an n-type electrode 1101 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1112 is deposited on thecontact layer 1111. The electrodes 1101 and 1112 are subjected to anannealing process, and the optical cavity of the laser diode is formedby cleaving the structure thus formed.

[0323] In the laser diode of FIG. 13, it should be noted that thecladding layer 1107, the contact layer 1111 and the cap layer 1109achieve a lattice matching to the GaAsP mixed crystal layer of thecomposition GaAs_(0.6)P_(0.4).

[0324] In the GaInAsP active layer 1106 formed with lattice matchingwith the GaAs_(0.6)P_(0.4) mixed crystal composition, it is possible tochange the bandgap wavelength from 560 nm to 650 nm. Further, it is alsopossible to increase the range of optical wavelength by adopting aquantum-well structure or applying strain to the active layer 1106.Further, it is possible to realize an optical wavelength of the 630 nmband or 650 nm band by introducing As into the active layer.

[0325] In the case of the laser diode of the illustrated construction,the laser diode oscillated at the wavelength of 635 nm. In this case, amixed crystal of AlGaInAsP was used for the current blocking regions1110, and the As content was set to be 20% in atomic percent for thegroup V elements constituting the mixed crystal. As a result, theproblem of hillock formation was successfully suppressed and a flat andsmooth surface was obtained. Thereby, the leakage current associatedwith the hillocks was reduced, and the frequency of initial failure ofthe laser diode was also reduced. With the elimination of currentleakage path, the injected drive current was effectively confined intothe stripe region as a result of the current-confinement action of thepnp structure formed outside the ridge stripe.

[0326] Further, in view of the fact that an AlGaInAsP composition havinga smaller bandgap as compared with the active layer 1106 is used for thecurrent-blocking regions 1110, there occurs an optical absorption forthe higher mode optical radiation leaked from the stripe region in thelateral direction. Thereby, a waveguide loss is caused at the regionoutside the ridge stripe for the higher-mode optical radiation thatspreads into such a region outside the stripe ridge structure.Associated therewith, the fundamental mode optical radiation is alone iseffectively confined in the ridge stripe and there is formed arefractive-index waveguide structure characterized by a single peak. Thelaser diode thereby oscillates stably in the fundamental lateral modeeven when operated to provide a high output power.

[0327] [Fifth Embodiment]

[0328]FIG. 14 shows the construction of a laser diode according to afifth embodiment of the present invention.

[0329] Referring to FIG. 14, the laser diode is constructed on asubstrate 1202 of n-type GaAs carrying thereon a composition-gradedlayer 1203 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1203 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1203 is well established a smooth surface is realized byoptimizing the composition gradient.

[0330] Next, a buffer layer 1204 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1203, andcladding layer 1105 of n-type AlGaInAsP, an undoped active layer 1206 ofGaInAsP, a cladding layer 1207 of p-type AlGaInAsP, a spike-eliminationlayer 1208 of p-type GaInP, and a cap layer 1209 of p-type GaAsP, aregrown consecutively on the buffer layer 1204 by an MOCVD process.

[0331] After the formation of the cap layer 1209, an SiO₂ film isdeposited by a CVD process, followed by a photolithographic process toform a stripe pattern in correspondence to the region where injection ofelectric current is to be made, with a width of 10 μm.

[0332] Next, the layers 1209 and 1208 are patterned consecutively by achemical etching process while using the SiO₂ film thus formed as amask, wherein the chemical etching process is continued until theetching reaches a part of the semiconductor layer 1207. As a result, amesa ridge stripe is formed as represented in FIG. 14.

[0333] In the foregoing chemical etching process, the p-type GaAs caplayer 1209 is patterned while using a sulfuric acid etchant, while thep-type GaInP layer 1208 and the p-type AlGaInAsP layer 1207 arepatterned by a hydrochloric acid etchant. The depth of etching of thecladding layer 1207 is controlled by way of controlling the duration ofthe etching process. According to such a process, it is possible tosimplify the fabrication process and device structure.

[0334] Next, a pair of current-blocking regions 1210 of n-type AlGaInAsPare formed on the mesa structure thus formed by a regrowth process witha composition set so as to achieve lattice matching with theGaAs_(0.6)P_(0.4) mixed crystal composition while using the SiO₂ film asthe mask covering the ridge region of the mesa structure, wherein thecurrent-blocking regions 1210 are grown on the region of the claddinglayer 1207 not covered by the SiO₂ mask. By introducing As with aconcentration of 5% into the current-blocking regions 1210, the problemof hillock formation was effectively suppressed.

[0335] Further, the SiO₂ mask is removed and a contact layer 1211 ofp-type GaAsP is grown on the current-blocking regions 1210 by a regrowthprocess so as to cover the p-type GaAsP cap layer 1209 exposed at theridge region of the mesa structure.

[0336] Thereafter, the bottom surface of the substrate 1202 is polishedand an n-type electrode 1201 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1212 is deposited on thecontact layer 1211. The electrodes 1201 and 1212 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0337] In the laser diode of FIG. 14, it should be noted that thecladding layer 1207, the current-blocking regions 1210, the contactlayer 1211 and the cap layer 1209 achieve lattice matching to the GaAsPmixed crystal layer of the composition GaAs_(0.6)P_(0.4).

[0338] In the GaInAsP active layer 1206 formed with lattice matchingwith the GaAs_(0.6)P_(0.4) mixed crystal composition, it is possible tochange the bandgap wavelength from 560 nm to 650 nm. Further, it is alsopossible to increase the range of optical wavelength by adopting aquantum-well structure or applying strain to the active layer 1206.Further, it is possible to realize an optical wavelength of the 630 nmband or 650 nm band by introducing As into the active layer 1206.

[0339] In the case of the laser diode of the illustrated construction,the laser diode oscillated at the wavelength of 635 nm. In this case, amixed crystal of AlGaInAsP was used for the current blocking regions1210 with the lattice matching composition to the GaAsP mixed crystalwhile setting the As content to 5% in atomic percent for the group Velements constituting the mixed crystal. Thereby, the refractive indexof the current-blocking regions 1210 is reduced as compared with therefractive index of the cladding layer 1207, and there is formed a realrefractive index waveguide structure.

[0340] Associated therewith, the efficiency of laser oscillation isimproved and the laser diode can operate stably with high optical outputpower. The use of the real-refractive index waveguide structure alsoreduces astigmatism of the output optical beam.

[0341] Further, the problem of hillock formation was successfullysuppressed and a flat and smooth surface was obtained by introducing 5%of As into the current-blocking regions 2210. Thereby, the leakagecurrent associated with the hillocks was reduced, and the frequency ofinitial failure of the laser diode was also reduced. With theelimination of current leakage path, the injected drive current waseffectively confined into the stripe region as a result of thecurrent-confinement action of the pnp structure formed outside the ridgestripe.

[0342] [Sixth Embodiment]

[0343]FIG. 15 shows the construction of a laser diode according to asixth embodiment of the present invention.

[0344]FIG. 15 shows the construction of a laser diode according to asixth embodiment of the present invention.

[0345] Referring to FIG. 15, the laser diode is constructed on asubstrate 1302 of n-type GaAs carrying thereon a composition-gradedlayer 1303 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1303 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1303 is well established a smooth surface is realized byoptimizing the composition gradient.

[0346] Next, a buffer layer 1304 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1303, andcladding layer 1305 of n-type AlGaInAsP, an undoped active layer 1306 ofGaInAsP, a first cladding layer 1307 of p-type AlGaInAsP, an etchingstopper layer 308 of p-type GaAsP, a second cladding layer of p-typeAlGaInAsP, a spike elimination layer 1310 of p-type GaInP, and a caplayer 1311 of p-type GaAsP, are grown consecutively on the buffer layer1304 by an MOCVD process.

[0347] After the formation of the cap layer 1311, an SiO₂ film isdeposited by a CVD process, followed by a photolithographic process toform a stripe pattern in correspondence to the region where injection ofelectric current is to be made, with a width of 10 μm.

[0348] Next, the layers 1311, 1310 and 1309 are patterned consecutivelyby a chemical etching process while using the SiO₂ film thus formed as amask, wherein the chemical etching process is continued until theetching stopper layer 1308 is exposed. As a result, a mesa ridge stripeis formed as represented in FIG. 14.

[0349] In the foregoing chemical etching process, the p-type GaAsP caplayer 1311 is patterned while using a sulfuric acid etchant, while thep-type GaInP layer 1310 and the p-type AlGaInAsP layer 1309 arepatterned by a hydrochloric acid etchant. As a result of use of theetching stopper layer 1308, the control of height of the ridge structureis substantially facilitated.

[0350] Next, a pair of current-blocking regions 1312 of n-type AlGaInAsPare formed on the mesa structure thus formed by a regrowth process whileusing the SiO₂ film as the mask covering the ridge region of the mesastructure, wherein the current-blocking regions 1312 are grown on theregion of the cladding layer 1312 not covered by the SiO₂ mask.

[0351] Further, the SiO₂ mask is removed and a contact layer 1313 ofp-type GaAsP is grown on the current-blocking regions 1312 by a regrowthprocess so as to cover the p-type GaAsP cap layer 1311 exposed at theridge region of the mesa structure.

[0352] Thereafter, the bottom surface of the substrate 1302 is polishedand an n-type electrode 1301 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1314 is deposited on thecontact layer 1313. The electrodes 1301 and 1314 are subjected to anannealing process so as to form an ohmic contact, and the optical cavityof the laser diode is formed by cleaving the structure thus formed.

[0353] In the laser diode of FIG. 15, a laser oscillation was obtainedwith the horizontal lateral mode at the wavelength of 650 nm.

[0354] As a result of use of the mixed crystal containing As for thecurrent-confinement regions 1312, the problem of hillock formation wassuccessfully eliminated. Thereby, the problem of leakage current orwaveguide loss associated with optical scattering is eliminated and thethreshold current of laser oscillation is reduced. In the illustratedexample, a composition of GaAs_(0.4)P_(0.6) was used for the etchingstopper layer 1308 so as to form a lattice misfit of about −0.73%.Thereby, the bandgap of the etching stopper layer 1308 exceeds thephoton energy of the laser beam radiation produced by the laser diodeand the problem of optical loss is avoided. It should be noted that theetching stopper layer 1308 is provided with a thickness less than thecritical thickness and the problem of degradation of crystal quality isavoided.

[0355] In the present embodiment that uses GaAsP for the etching stopperlayer 1308, the bandgap energy is larger than the case of using GaInPfor the etching stopper layer 1308. On the other hand, the latticestrain of the etching stopper layer 1308 can be minimized by using acomposition of GaInAsP. The etching stopper layer 1308 having such acomposition avoids optical absorption simultaneously.

[0356] As a result of use of the etching stopper layer 1308, it becomespossible, in the present embodiment, to apply an etching process to theregion where the active layer 1306 is provided or to the region in thevicinity of the active layer 1306, without causing an over-etching ofthe active layer 1506. Even so, the effect of non-optical surface stateson the etching surface was eliminated with regard to the devicecharacteristic or scattering of device characteristic.

[0357] Further, in view of the fact that the etching stopper layer 1308is covered with the cladding layer 1307, there occurs no surfaceoxidation, and the current-confinement regions 312 are formed thereonwith excellent crystal quality. As a result, the laser diode of thepresent embodiment shows little aging and operates with excellentreliability.

[0358] [Seventh Embodiment]

[0359]FIG. 16 shows the construction of a laser diode according to aseventh embodiment of the present invention.

[0360] Referring to FIG. 16, the laser diode is constructed on asubstrate 1402 of n-type GaAs carrying thereon a composition-gradedlayer 1403 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1403 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1403 is well established a smooth surface is realized byoptimizing the composition gradient.

[0361] Next, a buffer layer 1404 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1403, andcladding layer 1405 of n-type AlGaInAsP, an optical waveguide layer 1406of undoped GaInP, an active layer 1407 of undoped GaInAsP, an opticalwaveguide layer 1408 of undoped GaInP, a cladding layer 1409 of p-typeAlGaInAsP, a spike-elimination layer 1410 of p-type GaInP, and a caplayer 1411 of p-type GaAsP, are grown consecutively on the buffer layer1404 by an MOCVD process.

[0362] After the formation of the cap layer 1411, an SiO₂ film isdeposited by a CVD process, followed by a photolithographic process toform a stripe pattern in correspondence to the region where injection ofelectric current is to be made, with a width of 10 μm.

[0363] Next, the layers 1411, 1410 and 1409 are patterned consecutivelyby a chemical etching process while using the SiO₂ film thus formed as amask, wherein the chemical etching process is continued until theoptical waveguide layer 1408 is exposed. As a result, a mesa ridgestripe is formed as represented in FIG. 16.

[0364] In the foregoing chemical etching process, the p-type GaAsP caplayer 1411 is patterned while using a sulfuric acid etchant, while thep-type GaInP layer 1410 and an upper part of the p-type AlGaInAsP layer1409 are patterned by a hydrochloric acid etchant. Then the etchant ischanged again to the sulfuric acid etchant and the remaining part of theAlGaInAsP cladding layer 1409 is etched until the optical waveguidelayer 1408 is exposed. Thereby, the optical waveguide layer 1408 is usedas the etching stopper. As a result of use of the etching stopper, thepresent invention can control the height of the ridge structure easily.

[0365] Next, a pair of current-blocking regions 1413 of p-type AlGaInAsPare formed on the mesa structure thus formed by a regrowth process whileusing the SiO₂ film as the mask covering the ridge region of the mesastructure, wherein the current-blocking regions 1412 are grown on theregion of the optical waveguide layer 1408 and the cladding layer 1409not covered by the SiO₂ mask. Further, n-type AlInAsP current blockingregions 1413 are grown on the p-type current-blocking regions 1412 whileusing the SiO2 mask, similarly to the process of forming thecurrent-blocking regions 1412.

[0366] Further, the SiO₂ mask is removed and a contact layer 1414 ofp-type GaAsP is grown on the current-blocking regions 1413 by a regrowthprocess so as to cover the p-type GaAsP cap layer 1411 exposed at theridge region of the mesa structure.

[0367] Thereafter, the bottom surface of the substrate 1402 is polishedand an n-type electrode 1401 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1412 is deposited on thecontact layer 1414. The electrodes 1401 and 1415 are subjected to anannealing process, and the optical cavity of the laser diode is formedby cleaving the structure thus formed.

[0368] In the laser diode of FIG. 16, it should be noted that thecladding layer 1409, the contact layer 1414 and the current-blockingregions 1412 and 1413 form together a pnp structure acting as a currentconfinement structure.

[0369] In the illustrated example, the laser diode oscillated with thefundamental lateral mode at the wavelength of 640 nm.

[0370] By adding As into the mixed crystal layer constituting thecurrent-blocking regions 412 and 413, the problem of hillock formationis successfully eliminated in the laser diode of the present embodiment.Associated with this, the leakage current path is eliminated and thewaveguide loss caused as a result of optical scattering is minimized.

[0371] As the active layer 1407 is sandwiched by the GaInP opticalwaveguide layers 1406 and 1408, which is free from Al, non-opticalrecombination of carriers is reduced and the threshold of laseroscillation is reduced. Further, as a result of use of Al-free materialin the optical waveguide of the laser diode in which the opticalintensity is strong, the surface states associated with oxidation of Alis minimized and the COD level is increased. Thereby, the laser diodeoperates at a high optical output power.

[0372] [Eighth Embodiment]

[0373]FIG. 17 shows the construction of a laser diode according to aneighth embodiment of the present invention.

[0374] Referring to FIG. 17, the laser diode is constructed on asubstrate 1502 of n-type GaAs carrying thereon a composition-gradedlayer 1503 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1503 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1503 is well established a smooth surface is realized byoptimizing the composition gradient.

[0375] Next, a buffer layer 1504 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1503, and acladding layer 1505 of n-type AlGaInAsP, an undoped active layer 1506 ofGaInAsP, a first cladding layer 1507 of p-type AlGaInAsP, and a currentconfinement layer 1508 of n-type AlGaInAsP, are grown consecutively onthe buffer layer 1504 by an MOCVD process.

[0376] After the formation of the current confinement layer 1508, aresist film is deposited by a spin-coating process, followed by aphotolithographic process to form a stripe window in correspondence tothe region where injection of electric current is to be made, with awidth of 10 μm.

[0377] Next, the current confinement layer 1508 is patterned by achemical etching process while using the resist film thus formed as amask, wherein the chemical etching process is continued until theetching reaches the optical waveguide layer 1507. As a result, a stripegroove is formed as represented in FIG. 17. The chemical etching processmay be conducted by using a sulfuric acid etchant. As a result of thechemical etching process, a pair current-blocking regions 1508 areformed with an intervening stripe groove region exposing the opticalwaveguide layer 1507.

[0378] Next, the resist film is removed and a second cladding layer 1509of p-type AlGaInAsP, a spike-elimination layer 1510 of p-type GaInP, anda contact layer 1511 of p-type GaAsP are grown consecutively on thecurrent-blocking regions 1508 by a regrowth process so as to cover theoptical waveguide layer 1507 exposed at the stripe groove region.

[0379] Thereafter, the bottom surface of the substrate 1502 is polishedand an n-type electrode 1501 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1512 is deposited on thecontact layer 1511. The electrodes 1501 and 1512 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0380] In the laser diode of FIG. 17, it should be noted that thecladding layer 1509 and the contact layer 1111 achieve a latticematching to the GaAsP mixed crystal layer of the compositionGaAs_(0.6)P_(0.4).

[0381] In the case of the laser diode of the illustrated construction,the laser diode oscillated with the fundamental lateral mode at thewavelength of 635 nm.

[0382] It should be noted that the current-blocking layer or regions1508 contain As with the concentration of 20%. As a result, there occursno substantial formation of hillocks and a smooth and flat surface isobtained for the layer 1508. Thereby, the problem of leakage currentinduced by hillocks or the associated problem of initial failure of thelaser diode is effectively eliminated.

[0383] Further, in view of elimination of the leakage current path, itbecomes possible to confine the electric current into the stripe regionmore efficiently.

[0384] It should be noted that the fabrication process of the laserdiode of the present embodiment requires only two MOCVD process,contrary to the case of forming the laser diode having a stripe ridgestructure, which requires three separate MOCVD process. Thereby, thenumber of intermission of the growth process is reduced and degradationof quality of the epitaxial layers grown on such a surface is minimized.

[0385] [Ninth Embodiment]

[0386]FIG. 18 shows the construction of a laser diode according to aneighth embodiment of the present invention.

[0387] Referring to FIG. 18, the laser diode is constructed on asubstrate 1602 of n-type GaAs carrying thereon a composition-gradedlayer 1603 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1603 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1603 is well established a smooth surface is realized byoptimizing the composition gradient.

[0388] Next, a buffer layer 1604 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1603, and acladding layer 1605 of n-type AlGaInAsP, an undoped active layer 1606 ofGaInAsP, a first cladding layer 1607 of p-type AlGaInAsP, and a currentconfinement layer 1608 of n-type AlGaInAsP, are grown consecutively onthe buffer layer 1604 by an MOCVD process.

[0389] After the formation of the current confinement layer 1608, aresist film is deposited by a spin-coating process, followed by aphotolithographic process to form a stripe window in correspondence tothe region where injection of electric current is to be made, with awidth of loom.

[0390] Next, the current confinement layer 1608 is patterned by achemical etching process while using the resist film thus formed as amask, wherein the chemical etching process is continued until theetching reaches the optical waveguide layer 1607. As a result, a stripegroove is formed as represented in FIG. 17. The chemical etching processmay be conducted by using a sulfuric acid etchant. As a result of thechemical etching process, a pair current-blocking regions 1508 areformed with an intervening stripe groove region exposing the opticalwaveguide layer 1607.

[0391] Next, the resist mask is removed and a second cladding layer 1609of p-type AlGaInAsP, a spike-elimination layer 1610 of p-type GaInP, anda contact layer 1611 of p-type GaAsP are grown on the current-blockingregions 1608 consecutively by a regrowth process so as to cover theoptical waveguide layer 1607 exposed at the stripe groove region.

[0392] Thereafter, the bottom surface of the substrate 1602 is polishedand an n-type electrode 1601 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1612 is deposited on thecontact layer 1611. The electrodes 1601 and 1612 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0393] In the laser diode of FIG. 18, it should be noted that theAlInAsP current-blocking layer 1608, and hence the current-blockingregions 1608, is formed to have a composition transparent to the laserbeam radiation produced by the laser diode, by introducing 5% of As intothe composition of AlInP. Thereby, the current-blocking regions 1608achieve lattice matching with the composition GaAs_(0.6)P_(0.4).

[0394] With this amount of As, it was observed that hillock formation iseffectively suppressed. Further, it should be noted that the AlInAsPcurrent-blocking regions 1608 of the foregoing lattice matchingcomposition have a refractive index smaller than the refractive index ofthe cladding layer 1607. Thus, there occurs no substantial waveguideloss, and the threshold current is reduced further. Further, the outerdifferential quantum efficiency is improved and the laser diode canproduce high output optical power. In addition, the use of thereal-refractive index waveguide structure reduces the astigmatism of theoutput optical beam, and a single peak beam spot is obtained. The laserdiode causes an oscillation with the fundamental lateral mode whendriven to produce a high output optical power.

[0395] [Tenth Embodiment]

[0396]FIG. 19 shows the construction of a laser diode according to atenth embodiment of the present invention.

[0397] Referring to FIG. 19, the laser diode is constructed on asubstrate 1702 of n-type GaAs carrying thereon a composition-gradedlayer. 1703 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1703 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1703 is well established a smooth surface is realized byoptimizing the composition gradient.

[0398] Next, a buffer layer 1704 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1703, and acladding layer 1705 of n-type AlGaInAsP, an active layer 1706 of undopedGaInAsP, a first cladding layer 1707 of p-type AlGaInAsP, an etchingstopper layer 1708 of p-type GaInAsP, and a current confinement layer1709 of n-type AlGaInAsP, are grown consecutively on the buffer layer1704 by an MOCVD process.

[0399] After the formation of the current confinement layer 1709, aresist film is deposited by a spin-coating process, followed by aphotolithographic process to form a stripe window in correspondence tothe region where injection of electric current is to be made, with awidth of 10 μm.

[0400] Next, the current confinement layer 1709 is patterned by achemical etching process while using the resist film thus formed as amask, wherein the chemical etching process is conducted by using ahydrochloric acid etchant and continued until the etching stopper layer1708 is exposed. As a result, a stripe groove is formed as representedin FIG. 19. As a result of the use of the etching stopper layer 1708,the depth of the stripe groove is controlled exactly.

[0401] Next, the resist mask is removed and a second cladding layer 1710of p-type AlGaInAsP, a spike-elimination layer 1711 of p-type GaInP, anda contact layer 1712 of p-type GaAsP are grown consecutively on thecurrent-blocking regions 1709 by a regrowth process so as to cover theetching stopper layer 1708 exposed at the stripe groove region.

[0402] Thereafter, the bottom surface of the substrate 1702 is polishedand an n-type electrode 1701 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1713 is deposited on thecontact layer 1712. The electrodes 1701 and 1713 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0403] In the case of the laser diode of the illustrated example, thelaser diode oscillated with the fundamental lateral mode at thewavelength of 635 nm.

[0404] It should be noted that the current-blocking regions 1709 containAs. Thus, there occurs no substantial formation of hillocks and a smoothand flat surface is obtained for the layer 1709 and the layers grownthereon. Thereby, the problem of leakage current induced by hillocks iseffectively eliminated. Further, the problem of optical loss associatedwith the hillocks in the optical waveguide region is eliminated.

[0405] In the present invention, it should be noted that the GaInAsPetching stopper 1708 has a lattice-matching composition in which theetching stopper layer 1708 achieves lattice matching with the substrate.At this lattice-matching composition, the GaInAsP etching stopper layer1708 has a bandgap energy of about 2.0 eV, while this value of bandgapenergy exceeds the photon energy of the laser oscillation radiation.Further, in view of the fact that the etching stopper layer 1708achieves lattice matching, there occurs no limitation with regard to thethickness of the etching stopper layer 1708, and a desirable largeprocess margin can be secured for the etching process, by using a largethickness for the etching stopper layer 17.

[0406] By providing the etching stopper layer 1708, it becomes possibleto continue the etching process to the active layer 1706 or the regionin the vicinity of the active layer 1706, without causing over-etchingof the active layer 1706.

[0407] The laser diode of the present embodiment has an advantageousfeature in that the effect of non-optical recombination center such assurface state is minimized because of the excellent quality of thecrystal layers constituting the laser diode and excellent efficiency oflaser oscillation is realized. Further, device-to-device variation ofthe laser characteristic is also minimized. It should be noted that thefirst cladding layer 1707 of AlGaInAsP is covered by the p-type GaAsPetching stopper layer 1708. Thus, the first cladding layer 1707 remainsintact even when the etching process is conducted. Thus, the surface ofthe first cladding layer 1707 is free from surface oxidation or damages,and the current-blocking layer 1709 is grown thereon with excellentquality.

[0408] [Eleventh Embodiment]

[0409]FIG. 20 shows the construction of a laser diode according to aneleventh embodiment of the present invention.

[0410] Referring to FIG. 20, the laser diode is constructed on asubstrate 1802 of n-type GaAs carrying thereon a composition-gradedlayer 1803 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1803 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1803 is well established a smooth surface is realized byoptimizing the composition gradient.

[0411] Next, a buffer layer 1804 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1803, and acladding layer 1805 of n-type AlGaInAsP, an active layer 1806 of undopedGaInAsP, a first cladding layer 1807 of p-type AlGaInAsP, an etchingstopper layer 1808 of p-type GaInAsP, a current confinement layer 1809of n-type AlGaInAsP, and further an oxidation-prevention layer 1810 ofp-type GaInP, are grown consecutively on the buffer layer 1804 by anMOCVD process.

[0412] After the formation of the oxidation-prevention layer 1810, aresist film is deposited by a spin-coating process, followed by aphotolithographic process to form a stripe window in correspondence tothe region where injection of electric current is to be made, with awidth of 10 μm.

[0413] Next, the oxidation-prevention layer 1810 of GaInP and theunderlying current-blocking layer 1809 of AlInAsP are patterned by achemical etching process while using the resist film thus formed as amask, wherein the chemical etching process is conducted by using ahydrochloric acid etchant and is continued until the etching stopperlayer 1808 is exposed. As a result, a stripe groove is formed asrepresented in FIG. 20.

[0414] In the foregoing patterning process, the GaInPoxidation-prevention layer 1810 and the AlInAsP current-blocking layer1809 are patterned selectively with respect to the GaAsP etching stopperlayer 1808 by using a hydrochloric acid etchant, and a pair ofcurrent-blocking regions 1809 are formed from the current-blocking layer1809.

[0415] Next, the resist mask is removed and a second cladding layer 1811of p-type AlGaInAsP, a spike-elimination layer 1812 of p-type GaInP, anda contact layer 1813 of p-type GaAsP are grown consecutively on thecurrent-blocking regions 1809 by a regrowth process so as to cover theetching stopper layer 1808 exposed at the stripe groove region.

[0416] Thereafter, the bottom surface of the substrate 1802 is polishedand an n-type electrode 1801 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1813 is deposited on thecontact layer 1812. The electrodes 1801 and 1813 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0417] In the case of the laser diode of the illustrated example, thelaser diode oscillated with the fundamental lateral mode at thewavelength of 630 nm.

[0418] It should be noted that the current-blocking regions 1809 containAs. Thus, there occurs no substantial formation of hillocks and a smoothand flat surface is obtained for the layer 1809 and the layers grownthereon. Thereby, the problem of leakage current induced by hillocks iseffectively eliminated. Further, the problem of optical loss associatedwith the hillocks in the optical waveguide region is eliminated.

[0419] In the present embodiment, it should further be noted that thesurface oxidation of the AlInAsP current-blocking layer 1809 iseliminated by the oxidation-prevention layer 1810, and the secondcladding layer 1811 is grown thereon with excellent quality. Thus, thelaser diode of the present embodiment shows little aging and operatesreliably over a long time.

[0420] [Twelfth Embodiment]

[0421]FIG. 21 shows the construction of a laser diode according to atwelfth embodiment of the present invention.

[0422] Referring to FIG. 21, the laser diode is constructed on asubstrate 1902 of n-type GaAs carrying thereon a composition-gradedlayer 1903 of n-type GaAsP having a composition represented asGaAs_(y)P_(1-y), wherein the composition-graded layer 1903 is formed byan MOCVD process while changing the composition y continuously andgradually from 1 to 0.4. The growth process of the composition-gradedlayer 1903 is well established a smooth surface is realized byoptimizing the composition gradient.

[0423] Next, a buffer layer 1904 of n-type GaAsP having a composition ofGaAs_(0.6)P_(0.4) is grown on the composition-graded layer 1903, and acladding layer 1905 of n-type AlGaInAsP, an optical waveguide layer 1906of undoped GaInP, an active layer 1907 of undoped GaInAsP, an opticalwaveguide layer 1908 of undoped GaInP, a first current-blocking layer1909 of p-type AlGaInAsP, a second current-blocking layer 1910 of n-typeAlGaInAsP, and an oxidation-prevention layer 1911 of p-type GaInP, aregrown consecutively on the buffer layer 1904 by an MOCVD process.

[0424] After the formation of the oxidation-prevention layer 1911, aresist film is deposited by a spin-coating process, followed by aphotolithographic process to form a stripe window in correspondence tothe region where injection of electric current is to be made, with awidth of 10 μm.

[0425] Next, the oxidation-prevention layer 1911 of GaInP and theunderlying current-blocking layers 1910 and 1909 of AlInAsP arepatterned by a chemical etching process while using the resist film thusformed as a mask similarly to the previous embodiment, wherein thechemical etching process is conducted until the optical waveguide layer1908 is exposed. As a result, a stripe groove is formed as representedin FIG. 21. Thereby, the optical waveguide layer 1908 functions as anetching stopper.

[0426] Next, the resist mask is removed and a second cladding layer 1912of p-type AlGaInAsP, a spike-elimination layer 1913 of p-type GaInP, anda contact layer 1914 of p-type GaAsP are grown consecutively on theoxidation-prevention layer 1911 by a regrowth process so as to cover theoptical waveguide layer 1908 exposed at the stripe groove region.

[0427] Thereafter, the bottom surface of the substrate 1902 is polishedand an n-type electrode 1901 is formed thereon by an evaporationdeposition process. Further, a p-type electrode 1915 is deposited on thecontact layer 1914. The electrodes 1901 and 1915 are subjected to anannealing process to form an ohmic contact, and the optical cavity ofthe laser diode is formed by cleaving the structure thus formed.

[0428] In the case of the laser diode of the illustrated example, thelaser diode oscillated with the fundamental lateral mode at thewavelength of 630 nm.

[0429] It should be noted that the current-blocking regions 1909 and1910 contain As. Thus, there occurs no substantial formation of hillocksand a smooth and flat surface is obtained for the layers 1909 and 1910and the layers grown thereon. Thereby, the problem of leakage currentinduced by hillocks is effectively eliminated. Further, the problem ofoptical loss associated with the hillocks in the optical waveguideregion is eliminated.

[0430] Further, in view of the fact that the layer adjacent to theactive layer is free from Al, non-optical recombination of carriers issuppressed and the threshold current of laser oscillation is reduced. Inview of the fact that the region of the laser diode where there isformed strong optical radiation is free from Al, the number of surfacestates at the cavity edge surface is reduced and the optical damaging atthe optical cavity edge surface is minimized.

[0431] In the foregoing embodiments a description was made with regardto the laser diode structure constructed on a graded GaAsP layer formedon a GaAs substrate. However, it is possible to construct the laserdiode on a GaP substrate or GaAsP substrate. Further, acomposition-graded layer of GaInP may be used in place of the GaAsPcomposition graded layer. Further, the composition-graded layer may beformed by a process other than a vapor phase epitaxial process.

[0432] [Thirteenth Embodiment]

[0433]FIG. 22 shows the construction a semiconductor light-emittingdevice according to a thirteenth embodiment of the present invention.

[0434] Referring to FIG. 22, the semiconductor light-emitting device isconstructed on a semiconductor substrate 2001 and includes an activelayer 2004 emitting optical radiation and semiconductor layers 2002 and2003 having a bandgap larger than a bandgap of the active layer and alattice constant intermediate between a lattice constant of GaP and alattice constant of GaAs, wherein the semiconductor layers 2002 and 2003are formed so as to vertically sandwich the active layer 2004.

[0435] In the semiconductor light-emitting device of FIG. 22, thesemiconductor layer 2003 includes, in a part thereon, a layer 2005having a composition represented as Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t)(0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), wherein a part of the layer 2005 is convertedinto oxidized regions 2007 as a result of selective oxidation.

[0436] In the semiconductor light-emitting device of FIG. 22, theforegoing oxidized regions 2007 become an insulator, and thus, thesemiconductor layer 2005 functions as a current-confinement structure.As the oxidized regions 2007 have a reduced refractive index, thereoccurs a refractive index step between the unoxidized part of thesemiconductor layer 2005 and the oxidized regions 2007. As a result,there emerges a real-refractive index waveguide structure suitable forlateral mode control.

[0437] Further, the structure of FIG. 22 is suitable for increasing theoutput power in view of the fact that the waveguide structure in thevicinity of the active layer 2004 is formed of a material free fromwaveguide loss.

[0438] It should be noted that, in the prior art device, it has beennecessary to realize such a real-waveguide structure by using a buriedstructure, which requires a number of crystal growth processes. Contraryto the prior art, the structure of FIG. 22 can be formed by a singlecrystal growth process. Thereby, the semiconductor light-emitting deviceof the present embodiment can be formed easily with high yield ofproduction.

[0439] [Fourteenth Embodiment]

[0440]FIG. 23 shows the construction of a semiconductor light-emittingdevice according to a fourteenth embodiment of the present invention,wherein those parts corresponding to the parts described with referenceto FIG. 22 are designated by the same reference numerals and thedescription thereof will be omitted.

[0441] Referring to FIG. 23, the semiconductor light-emitting device hasa structure similar to that of the device of FIG. 22 except that theactive layer 2004 is formed of a single quantum well structure or amultiple quantum well structure and that the active layer 2004 isvertically sandwiched by a pair of optical waveguide layers 2024 and2025 having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ<1, 0<u≦1),wherein the active layer 2004 has a composition represented as(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1, 0≦t₁≦1)when formed of a single quantum well. When the active layer 2004 isformed of a multiple quantum well structure, on the other hand, theactive layer 2004 is formed of alternate stacking of a quantum welllayer of the foregoing composition and a barrier layer of a compositionrepresented as (Al_(x2)Ga_(1-x2))_(α2)In_(1-α2)P_(t2)As_(1-t2) (0≦x₂<1,0.5<α₂≦1, 0≦t₂≦1). Further, each of the cladding layers 2002 and 2003has a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0≦y≦1, 0.5<β<1, 0<v≦1),wherein the composition of the cladding layers 2002 and 2003 is set suchthat the cladding layers 2002 and 2003 have a bandgap larger than abandgap of the active layer 2004 and a lattice constant between GaP andGaAs. The composition of the optical waveguide layers 2024 and 2025 isset such that the optical waveguide layers 2024 and 2025 have a bandgaplarger than the bandgap of the active layer 2004 but smaller than thebandgap of the cladding layers 2002 and 2003.

[0442] In the construction of FIG. 23 or 24, it should be noted that thesemiconductor light-emitting device includes, in one of the claddinglayers 2002 and 2003 (layer 2003 in the example of FIG. 23), or betweenone of the cladding layers 2002 or 2003 (layer 2003 in the example ofFIG. 24) and the active layer 2004, a semiconductor layer 2005 having acomposition represented as Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1,0≦y≦0.2, 0≦t≦1) is provided in such a manner that a part of the layer2005 is selectively oxidized to form oxidized regions 2007.

[0443] In the case of the device of FIG. 23, the active layer 2004,having the composition of(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1,0≦t₁≦1), is capable of emitting visible wavelength radiation. Further,the cladding layers 2002 and 2003, having the lattice constant betweenGaP and GaAs and the composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1), havea bandgap larger than the bandgap realized by a material formed on aGaAs substrate, and the device of FIG. 23 is suitable for producingshort wave optical radiation.

[0444] Further, in view of the fact that the optical waveguide layers2024 and 2025 of the composition(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ<1, 0<u≦1) forman SCH structure together with the active layer of the composition(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1, 0≦t≦1), thedevice of FIG. 23 can realize a wide bandgap with a reduced Al contentfor the optical waveguide layers 2024 and 2025, and the electric currentcaused as a result of non-optical recombination or carriers or surfacerecombination of carriers is reduced. As a result, the efficiency ofoptical emission is improved. In the case the device is a laser diode,the problem of degradation of the optical cavity edge is reduced and thelaser diode becomes operable under high-output power condition. In theconstruction of FIG. 23, it is also possible to introduce strain intothe cladding layer. Further, it is possible to reduce the bandgap of thecladding layers as compared with prior art devices.

[0445] It should be noted that a mixed crystal of GaInP increases thelattice constant and decreases the bandgap with decreasing Ga content.According to the estimation by Sandip, et al., Appl. Phys. Lett. 60,1992, pp.630-362 with regard to the band discontinuity, the banddiscontinuity increases primarily on the conduction band while thereoccurs no substantial change on the valence band. More specifically, thechange of band structure for the valence band is small even when thecomposition of the GaInP mixed crystal is changed. Further, there is atendency that the conduction band energy increases when Al is added to aGaInP mixed crystal. At the same time, the valence band energy isdecreased. Thereby, the magnitude of change of energy is much larger inthe valence band than in the conduction band energy. In relation to thissituation, there has been a drawback in a conventional semiconductorlight-emitting device constructed on a GaAs substrate in that, whilethere is formed a large band discontinuity on the conduction band, theband discontinuity on the valence band is not sufficient for effectivecarrier confinement.

[0446] The device structure of FIG. 23 is advantageous with this regardin that a large band discontinuity is secured for the conduction banddue to the decrease of the Al content in the optical waveguide layers2024 and 2025. Thereby, the problem of electron overflowing, which hasbeen a major problem in red-wavelength laser diodes of the system ofAlGaInP, is reduced substantially.

[0447] Further, as a result of formation of the insulating regions 2007,caused by the selective oxidation of the layer 2005 ofAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) containinga high concentration Al, there is formed a current-confinement structureby the insulating regions 2007 and the remaining part of the layer 2005having the composition of Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1,0≦y≦0.2, 0≦t≦1). In view of the fact that the selectively oxidizedinsulating regions 2007 have a refractive index smaller than therefractive index of the remaining part of the layer 2005 of thecomposition Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2,0≦t≦1), there is a lateral diffraction index step formed incorrespondence to the remaining part of the layer 2005, and there isformed a refractive-index waveguide structure that can be used forcontrolling the lateral mode. It should be noted that the part of thedevice in the vicinity of the active layer 2004 and constituting thewaveguide structure is formed of a material free from waveguide loss.Thus, the device of the present embodiment is suitable for producing ahigh optical output power. Conventionally, such an optical waveguidestructure free from optical loss has to be formed by repeating a numberof crystal growth steps. In the case of the present invention, on theother hand, it is possible to form the desired waveguide structure in asingle crystal growth process.

[0448] In the semiconductor light-emitting device of FIG. 22 or FIG. 23,it should be noted that the substrate 2001 is formed of GaAsP, and thesemiconductor layer 2005 of the composition represented asAl_(x)Ga_(y)In_(1-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) achieveslattice matching with the substrate 2001. It should be noted that such aGaAsP substrate 2001 can be formed by growing a GaPAs graded layerhaving a lattice constant between GaP and GaAs on one of a GaAssubstrate or a GaP substrate by an epitaxial process such as a vaporphase epitaxial process with a large thickness such as 50 μm such thatthe GaPAs composition changes gradually in the graded layer. Bycontrolling the composition of the graded layer such that the latticeconstant at the top part of the graded layer becomes identical with thelattice constant of the heterojunction part (at least the cladding layer2002), it becomes possible to form a heteroepitaxial system withoutinducing the problem of lattice misfit.

[0449] There is a tendency that the oxidation rate of the semiconductorlayer 2005 of the composition Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t)(0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) becomes small when the thickness of the layer2005 is small. Associated therewith, there is a possibility that theinsulator regions 2007 may be too small for an effectivecurrent-blocking layer. In the present invention, in which the layer2005 of the composition Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1,0≦y≦0.2, 0≦t≦1) achieves a lattice matching with the GaAsP substrate2001, it becomes possible to form the layer 2005 with a sufficientthickness. Thereby, a sufficient oxidation rate is secured for the layer2005 and the throughput of device fabrication process can be increased.

[0450] In the device of FIG. 22 or FIG. 23, it is possible to use a GaAssubstrate for the substrate 2001. In this case, the active layer 2004 issandwiched by semiconductor layers that have a lattice matchingcomposition with GaAs.

[0451] In the case of using GaAs for the substrate 2001, it is possibleto use AlAs for the layer 2005. In this case, however, there arises aproblem, due to the fact that the AlAs layer accumulates a compressivestrain of about 0.14%, that the active layer 2004 may be subjected to anadversary effect. By using the composition ofAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) for thesemiconductor layer 2005, it is possible to achieve a lattice matchingwith the GaAs substrate and the effect of strain is eliminated.

[0452] In the semiconductor light-emitting device of FIG. 22 or 23, thelayer 2005 of the foregoing compositionAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) is leftunoxidized for the region having a width w1 as the current path of thedevice, wherein the width w1 is set such that a ratio of w1 with respectto a quantity defined as the sum of the width w1 and the total width,represented as w2, of the oxidized regions 2007 (w1/(wl+w2)) is equal toor smaller than 0.6. When the foregoing ratio is larger than 0.6, thelight-emission can be caused in the region close to the edge of a ridgestructure, provided that such a ridge structure is formed in the deviceas represented by a broken line in FIG. 22 or 23. Thereby, there canoccur a waveguide loss as a result of fluctuation of the edge width.When the foregoing ratio w1/(w1+w2) is smaller, the effect of the edgewidth fluctuation is reduced and the device can operate with a largeroptical output power.

[0453] [Fifteenth Embodiment]

[0454]FIG. 24 shows the construction of a semiconductor light-emittingdevice according to a fifteenth embodiment of the present invention,wherein those parts corresponding to the parts described previously withreference to FIGS. 22 and 23 are designated by the same referencenumerals and the description thereof will be omitted.

[0455] Referring to FIG. 24, the semiconductor light-emitting device ofthe present embodiment has a construction similar to that of the deviceof FIG. 22 or 23, except that there is formed a ridge structure 2009having a width d as a part of the cladding layer 2003 locating above thesemiconductor layer 2005 of the composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1). In theillustrated example, the width d of the ridge structure 2009 is setequal to or larger than 10 μm.

[0456] In the construction of FIG. 24, in view of the fact that there isprovided the selectively oxidized regions 2007 underneath the ridgestructure 9, the ridge structure 9 itself can be formed with anincreased with without deteriorating the current confinement takingplace in the device. Because of the increased size of the ridgestructure 9, it is possible to form a electrode thereon with largecontact area, without providing a thermally insulating dielectric film.Thereby, the differential resistance of the device is minimized.Further, the structure is suitable for employing a junction-downmounting structure. In this case, the heat of the device is easilydissipated to a mounting substrate on which the device of FIG. 24 isflip-chip mounted.

[0457] [Sixteenth Embodiment]

[0458]FIG. 25 shows the construction of a light-emitting semiconductordevice according to a sixteenth embodiment of the present invention,wherein those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0459] Referring to FIG. 25, the light-emitting semiconductor device hasa construction similar to the device of FIG. 24 except that there isprovided-an etching stopper layer 2029 having a composition representedas Ga_(y)In_(1-y)P_(t)As_(1-t) (0<y≦1, 0≦t≦1) underneath thesemiconductor layer 2005 of the compositionAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1). Byproviding the etching stopper layer 2029, it becomes possible to controlthe height of the ridge structure 2009 exactly. Thereby, the fabricationof the semiconductor device is substantially facilitated.

[0460] In any of the foregoing embodiments of FIGS. 22-25, the layer2005 of the composition Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1,0≦y≦0.2, 0≦t≦1) may actually have a composition of AlP_(t)As_(1-t)(0≦t≦1). In this case, Al is the only group III elements constitutingthe layer 2005. In view of the increased oxidation rate of the AlGaInPAssystem, which is extremely sensitive to the Al content therein, itbecomes possible to reduce the time needed for fabricating thesemiconductor light-emitting device by using the AlPAs for thesemiconductor layer 2005. In the case other layers, such as the claddinglayer, contain Al with high concentration in view of the need ofincreasing the bandgap, such layer may also be oxidized together withthe layer 2005 when the Al content in the layer 2005 is close to the Alcontent in such a widegap layer. Thus, the use of the AlP_(t)As_(1-t)(0≦t≦1) composition is advantageous for forming a current-blockingstructure by way of selective oxidation.

[0461] [Sixteenth Embodiment]

[0462]FIG. 26 shows the construction of a semiconductor light-emittingdevice according to a seventeenth embodiment of the present invention.

[0463] Referring to FIG. 26, the semiconductor light-emitting device hasa structure similar to that of the device of FIG. 23 or FIG. 24 in thatthe active layer 2004 is vertically sandwiched by the cladding layers2002 and 2003.

[0464] In the structure of FIG. 26, it should be noted that a part ofthe cladding layer 2003 includes a layer 2015 of AlGaInAs having acomposition represented as Al_(x)Ga_(y)In_(1-x-y)As (0.8≦x≦1, 0≦y≦0.2),and a part of the layer 2015 is oxidized selectively to form a pair ofinsulator regions 2017, such that the insulator regions 2017 laterallysandwich an unoxidized region of the layer 2015 therebetween with thewidth of w1. Thereby, the width w1 is set such that the ratio w1/(w1+w2)is equal to or smaller than 0.6.

[0465] In the present embodiment, too, it should be noted that thematerials in the vicinity of the active layer 2004 are free from opticalabsorption with regard to the wavelength of the optical radiationproduced as a result of laser oscillation, and the semiconductorlight-emitting device can be produce a large output optical power.

[0466] Similarly to the embodiment of FIG. 22 or 23, there arises theproblem of optical waveguide loss in the structure of FIG. 26 when theforegoing ratio w1/(w1+w2) is larger than 0.6 due to the fluctuation ofedge width of the ridge structure, provided that a ridge structure isformed on the cladding layer 2003 as represented by a broken line inFIG. 26. By setting the ratio w1/(w1+w2) to be equal to or smaller than0.6, the forgoing problem of optical waveguide loss is successfullyeliminated.

[0467] [Eighteenth Embodiment]

[0468]FIG. 27 shows the construction of a semiconductor light-emittingdevice according to an eighteenth embodiment of the present invention,wherein those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0469] Referring to FIG. 27, the semiconductor light-emitting device hasa structure similar to the device of FIG. 24 in that the ridge structure2009 is formed on the structure of FIG. 26. Thereby, the ridge structure2009 is formed so as to cover the semiconductor layer 2015 including theinsulator regions 2017 with a width d set such that the width d exceeds10 μm.

[0470] In the present embodiment in which the semiconductor layer 2015is free from P, an effective current confinement is achieved. Thus, itbecomes possible to increase the width d of the ridge structure 2009 anda large contact area is secured for the electrode provided on the ridgestructure 2009. Further, in view of the fact that use of insulating filmis not necessary in the device of the present embodiment, thedifferential resistance of the device is reduced. In view of theincreased electrode area, the structure of FIG. 27 is suitable forflip-chip mounting on a support substrate, wherein such a flip-chipmounting is advantageous due to improved efficiency of heat dissipation.

[0471] [Nineteenth Embodiment]

[0472]FIG. 28 shows the construction of a semiconductor light-emittingdevice according to a nineteenth embodiment of the present invention,wherein the device of FIG. 28 is actually a laser diode having an SCH-QWstructure.

[0473] Referring to FIG. 28, the laser diode is constructed on a GaAsoffset-substrate 2111 having an inclined principal surface inclined fromthe (100) surface in the [110] direction with an offset angle of 2°.

[0474] On the substrate 2111, there is formed a composition-graded layer2112 of n-type GaPAs by a vapor phase epitaxial process such that the Pcontent increases gradually from 0 to 0.4. Thus, the GaPAscomposition-graded layer 2112 has a composition of GaP_(0.4)As_(0.6) onthe top part thereof. On the composition-graded layer 2112, a GaPAslayer 2113 having the foregoing composition of GaP_(0.4)As_(0.6) isformed such that the total thickness of the layers 2112 and 2113 becomesabout 50 μm. The layers 2112 and 2113 form, together with the GaAssubstrate 2111, a GaPAs epitaxial substrate 2101. Alternatively a GaPsubstrate may be used in place of the GaAs substrate 2111. Generally, aGaPAs substrate includes an epitaxial layer of GaPAs on a GaAs or GaPsubstrate with a thickness of 30 μm or more. At the surface of the GaPAslayer, the lattice misfit is sufficiently relaxed, and thus, thesubstrate 2101 formed of the GaAs substrate 2111 and the GaPAs layers2112 and 2113 can be regarded as a single GaPAs ternary substrate.

[0475] On the GaPAs substrate 2101, a cladding layer 2102 of n-typeAlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85) isformed by an MOCVD process with a thickness of 1 μm, wherein thecladding layer contains As and has the composition set so as to achievelattice matching with the GaP_(0.4)As_(0.6) substrate 2101.

[0476] On the cladding layer 2102, there is formed an optical waveguidelayer 2114 of p-type AlGaInPAs having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1) by anMOCVD process with a thickness of 0.1 μm, and a single quantum-wellactive layer 2104 of AlGaInPAs is formed on the optical waveguide layer2114 with a thickness of 25 nm by an MOCVD process with a compositionrepresented as (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.65,t=0.9), wherein the composition of the active layer 2104 is selected soas to accumulate a compressive strain therein.

[0477] Further, an optical waveguide layer 2115 of p-type AlGaInPAshaving a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1) is formedon the active layer 2104 by an MOCVD process with a thickness of 0.1 μm,and a first p-type cladding layer 2103 of p-type AlGaInPAs is formed onthe optical waveguide layer by an MOCVD process with a thickness of 0.1μm and a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85).

[0478] On the first p-type cladding layer 2103, there is formed a layer2105 of p-type AlGaInPAs layer by an MOCVD process with a thickness of50 nm such that the layer 2105 has a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (x=1, y=0, t=0.4), wherein thiscomposition is actually represented as AlP_(0.4)As_(0.6).

[0479] Further, a second p-type cladding layer 2106 is formed on theAlPAs layer 2105 by an MOCVD process with a thickness of about 0.9 μm,wherein the p-type cladding layer 2106 has a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85).Further, a buffer layer 2116 of p-type GaInP having a compositionrepresented as Ga0.7In0.3P and a contact layer 2117 of p-type GaPAshaving a composition represented as GaP_(0.4)As_(0.6) are grownconsecutively on the second p-type cladding layer 2106 with respectivethicknesses of 0.1 μm and 0.2 nm.

[0480] In the foregoing layered structure, it should be noted that thelayers 2102, 2103, 2114, 2115 and 2105 have respective compositionschosen so as to achieve a lattice matching with the GaPAs substrate2101. During the MOCVD process for forming the layered structure, TMG,TMI, TMA, AsH₃ and PH₃ may be used for the gaseous source together witha carrier gas of H₂.

[0481] After the formation of the layered structure, a photolithographicpatterning process is applied so as to remove a part of the layeredstructure in correspondence to a stripe region, until the AlPAs layer2105 of the composition AlP_(0.4)As_(0.6) is removed and the underlyingcladding layer 2103 is exposed. As a result of the photolithographicpatterning process, there is formed a ridge stripe structure 2109 suchthat the ridge stripe structure 2109 extends in an axial direction ofthe laser diode.

[0482] After formation of the ridge stripe structure 2109, thehalf-product of the laser diode thus obtained is subjected to anoxidation process conducted in a water vapor atmosphere at 450° C., andthere are formed oxidized regions 2107 such that each of the oxidizedregions 2107 penetrates into the ridge structure 2109 from a lateralside of the ridge stripe structure 2109 with a depth of 1.5 μm. Thereby,there remains a central, non-oxidized region of the layer 2105 with awidth of about 3 μm, wherein the unoxidized region form acurrent-confinement structure together with the oxidized regions 2107acting as a current-blocking region. As a result of formation of thecurrent-blocking structure in the layer 2105, a light-emission takesplace in correspondence to the region right underneath the unoxidizedregion of the AlPAs layer 2105. In the foregoing construction, it shouldbe noted that the ratio of the unoxidized region of the layer 2105 tothe width of the ridge stripe structure is about 0.5.

[0483] After forming the oxidized regions 2107 by the selectiveoxidizing process, an SiO₂ film 2118 is deposited so as to cover theridge structure, and a window is formed in correspondence to the ridgetop surface. Further, a p-type electrode 2119 is deposited on the SiO₂film 2118 in contact with the contact layer 2117 at the contact window.

[0484] The GaAs substrate 2111 is then subjected to a polishing processat the bottom surface thereof such that the thickness of the substrate2111 becomes 100 μm, and an n-type electrode 120 is deposited on thepolished bottom surface of the GaAs substrate 2111.

[0485] According to the present embodiment, a laser diode oscillating atthe wavelength of 660 nm is obtained.

[0486] As a result of the selective oxidation of the AlGaInPAs layer2105 containing Al with high concentration, a part of the layer 2105 isconverted into insulator in correspondence to the regions 2107, and theoxidized regions 2107 form the desired current-confinement structuretogether with the central unoxidized region of the AlGaInPAs layer 2105.

[0487] In view of the fact that the oxidized regions 2107 of theAlGaInPAs layer 2105, having a composition generally represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), has arefractive index smaller than a refractive index of the layer 2105itself, there is formed a refractive index profile in the layer 2105 andthe refractive index profile forms a real-refractive index waveguidestructure effective for lateral mode control. For example, it ispossible to control the lateral mode of laser oscillation by optimizingthe distance between the active layer 2104 and the layer 2105 ofAlGaInPAs of the foregoing general composition ofAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1).

[0488] In view of the fact that the laser diode uses a material freefrom optical absorption in the wavelength range corresponding to theoscillation wavelength of the laser diode, for the part in the vicinityof the active layer 2104. Thereby, the laser diode can produce a largeoptical output power.

[0489] In the fabrication process of the laser diode of FIG. 28, itshould be noted only a single regrowth process is necessary for formingthe desired current-confinement structure including the ridge stripestructure. In conventional laser diodes having a buried heterostructure,formation such a current-confinement structure requires a number ofregrowth process steps. Thus, the fabrication process of the laser diodeis simplified in the present embodiment and the fabrication of the laserdiode is facilitated. Associated with this, the yield of production ofthe laser diode is improved.

[0490] In the construction of FIG. 28, it should be noted that theoff-angle of the GaP_(0.4)As_(0.6) substrate 101 is small. As notedpreviously, the off-angle of only 2° is used in the construction of FIG.28. Thereby, the present embodiment successfully avoids the problem ofhillock formation, which is frequently observed in an AlGaInP layergrown by an MOCVD process on a substrate such as GaP, GaAs orGaP_(0.4)As_(0.6), for the case in which the substrate has a smalloff-angle. It should be noted that this tendency of hillock formationbecomes conspicuous when the Al content is increased. In the case of thelaser diode having a structure as shown in FIG. 28, the effect of thehillock formation on the device performance can become serious in viewof the use of large thickness for the cladding layers.

[0491] In the present embodiment, the problem of hillock formation issuccessfully avoided by introducing As into the layer of AlGaInP. Byincorporating As, the droplet formation of Al or Ga during the MOCVDprocess of the AlGaInP layer is suppressed. Thereby, the hillockformation is successfully suppressed even in such a case the off-angleof the substrate 2101 is set small.

[0492] The laser diode of FIG. 28 has another advantageous feature inthat Al content can be reduced as compared with a conventional materialformed on a GaAs substrate while maintaining the same bandgap. Forexample, the Al content in the optical waveguide layers 2114 and 2115 isreduced as compared with a conventional optical waveguide layer, and thecurrent associated with non-optical recombination of carriers isreduced. Thereby, the efficiency of light-emission is improved. Further,in view of the fact that the surface recombination current is alsoreduced and the degradation of optical cavity edge surface is reduced atthe same time, the output power of the laser diode can be increased ascompared with a conventional laser diode. Thus, the laser diode of thepresent embodiment can be used for a high-power red-wavelength laserdiode under a high temperature environment.

[0493] In the laser diode of FIG. 28, it should be noted that the activelayer 2004 of the single quantum well structure can be replaced with amultiple quantum well structure. In this case, the quantum well layerrepeated alternately together with a barrier layer having a compositionrepresented as (Al_(x2)Ga_(1-x2))_(α2)In_(-α2)P_(t2)As_(1-t2) (0≦x₂<1,0.5<α₂<1, 0≦t₂≦1). It should be noted that the optical waveguide layers2114 and 2115 may contain As.

[0494] In the present embodiment, it should be noted that theto-be-oxidized layer 2105 of p-type Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t)(x=1, y=0 and t=0.4) may be replaced with a p-type AlAs layer. In thiscase, the compositional parameters y and t are set to zero (y=t=0) whilethe compositional parameter x is set to one (x=1). When this compositionis used, the layer 2105 accumulates a compressive strain of about 1.4%.Thus, there is a limitation in the thickness of the layer 2105 and thelayer 2105 is formed to have a thickness of about 20 nm. It was foundthat the oxidation rate of an AlAs layer is larger than the oxidationrate of an AlPAs layer of the same thickness but contains P. The growthof a binary mixed crystal layer of AlAs is much easier than growing aternary mixed crystal layer of AlPAs.

[0495] [Twentieth Embodiment]

[0496]FIG. 29 shows the construction of a semiconductor optical deviceaccording to a twentieth embodiment of the present invention, whereinthe device of FIG. 29 is actually a laser diode having an SCH-MQWstructure.

[0497] Referring to FIG. 29, the laser diode is constructed on a GaAsoffset-substrate 2131 having an inclined principal surface inclined fromthe (100) surface in the [110] direction with an offset angle of 2°.

[0498] On the substrate 2131, there is formed a composition-graded layer2132 of n-type GaPAs by a vapor phase epitaxial process such that the Pcontent increases gradually from 0 to 0.4. Thus, the GaPAscomposition-graded layer 2132 has a composition of GaP_(0.4)As_(0.6) onthe top part thereof. On the composition-graded layer 2132, a GaPAslayer 2133 having the foregoing composition of GaP_(0.4)As_(0.6) isformed such that the total thickness of the layers 2132 and 2133 becomesabout 90 μm. The GaPAs layers 2132 and 2133 form a GaPAs substrate 2121together with the GaAs substrate 2131.

[0499] On the GaPAs substrate 2121, a cladding layer 2122 of n-typeAlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85) isformed by an MOCVD process with a thickness of 1 μm, wherein thecladding layer contains As and has the composition set so as to achievelattice matching with the GaP_(0.4)As_(0.6) substrate 2121.

[0500] On the cladding layer 2122, there is formed an optical waveguidelayer 2134 of p-type AlGaInPAs having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7, u=1) by an MOCVDprocess with a thickness of 0.1 μm, and a quantum-well layer ofAlGaInPAs having thickness of about 10 nm and a composition representedas (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.65, t=0.9) and abarrier layer of AlGaInPAs having a thickness of 10 nm and a compositionrepresented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7,u=1) are repeated alternately on the optical waveguide layer 2134 toform an active layer 2124 of a multiple quantum well structure, whereinthe composition of the quantum well layer is selected so as toaccumulate a compressive strain therein.

[0501] Further, an optical waveguide layer 2135 of p-type AlGaInPAshaving a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1) is formedon the active layer 2124 with a thickness of 0.1 μm, and a first p-typecladding layer 2123 of p-type AlGaInPAs is formed on the opticalwaveguide layer 2135 with a thickness of 0.1 μm and a compositionrepresented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8,v=0.85).

[0502] On the first p-type cladding layer 2123, there is formed a layer2125 of p-type AlGaInPAs layer with a thickness of 50 nm such that thelayer 2125 has a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (x=1, y=0, t=0.4), wherein thiscomposition is actually represented as AlP_(0.4)As_(0.6).

[0503] Further, a second p-type cladding layer 2126 is formed on theAlPAs layer 2125 with a thickness of about 0.9 μm, wherein the p-typecladding layer 2126 has a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85).Further, a buffer layer 2136 of p-type GaInP having a compositionrepresented as Ga_(0.7)In₀₃P and a contact layer 2137 of p-type GaPAshaving a composition represented as GaP_(0.4)As_(0.6) are grownconsecutively on the second p-type cladding layer 2126 with respectivethicknesses of 0.1 μm and 0.2 nm.

[0504] In the foregoing layered structure, it should be noted that thecladding layers 2122, 2123 and 2126, the optical waveguide layers 2134and 2135, and the layer 2125 achieve a lattice matching with the GaPAssubstrate 2121.

[0505] Next, the layered structure thus obtained is subjected to aphotolithographic patterning process to form a central ridge stripestructure, wherein the ridge stripe structure used in the embodiment ofFIG. 29 has an increased width of 50 μm as compared with the previousembodiment of FIG. 28. Thereby, the etching process is continued untilthe cladding layer 2123 underneath the layer 2125 is exposed. Further,an oxidation process is conducted in a water vapor atmosphere at thetemperature of 450° C. to cause an oxidation in the AlP_(0.4)As_(0.6)layer 2125. Thereby, the oxidation starts at the exposed edge of thelayer 2125 and proceeds to the interior of the ridge stripe structurealong the layer 2125, and a pair of oxidized regions 2127 are formed asa result such that each oxidized region 2127 extends into the interiorof the ridge stripe structure from a side wall thereof along the layer2125 with a distance of about 22.5 μm. Thereby, a region of unoxidizedAlPAs layer 2125 is left at the center of the two oxidized regions 2127with a width of 5 μm, wherein this unoxidized region provides thecurrent path of the drive current. On the other hand, the oxidizedregions 2127 function as a current-blocking regions and there is formeda current-confinement structure in the ridge stripe structure by theunoxidized part of the AlPAs layer 2125 and the oxidized regions 2127.In correspondence to the injection of the drive current via theunoxidized part of the layer 2125, there occurs a light emission rightunderneath the unoxidized part of the layer 2125. In the presentembodiment, the ratio of the width of the unoxidized part to the entirewidth of the ridge stripe structure is about 0.1.

[0506] After formation of the ridge stripe structure, the lateral sidesof the ridge stripe structure are filled with a polyimide as representedby regions 2128 and a p-type electrode 2138 is formed on the top part ofsuch a planarized structure in contact with the contact layer 2137.Further, the bottom surface of the GaAs substrate 2131 is polished to athickness of 100 μm, and an n-type electrode 2139 is formed on such apolished bottom surface.

[0507] According to the construction of FIG. 29, a laser diodeoscillating at the wavelength of 650 nm is obtained.

[0508] As a result of the selective oxidation of the AlGaInPAs layer2125 containing Al with high concentration, a part of the layer 2125 isconverted into insulator in correspondence to the regions 2127, and theoxidized regions 2127 form the desired current-confinement structuretogether with the central unoxidized region of the AlGaInPAs layer 2125.

[0509] In view of the fact that the oxidized regions 2127 of theAlGaInPAs layer 2125, having a composition generally represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), has arefractive index smaller than a refractive index of the layer 2125itself, there is formed a refractive index profile in the layer 2125 andthe refractive index profile forms a real-refractive index waveguidestructure effective for lateral mode control. For example, it ispossible to control the lateral mode of laser oscillation by optimizingthe distance between the active layer 2124 and the layer 2125 ofAlGaInPAs of the foregoing general composition ofAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1).

[0510] As the waveguide structure is formed inside the ridge stripe witha sufficient distance from the side wall of the ridge stripe structure,the laser diode of the present embodiment successfully minimizes thewaveguide loss associated with the fluctuation of the edge width.

[0511] In the fabrication process of the laser diode of FIG. 29, itshould be noted only a single regrowth process is necessary for formingthe desired current-confinement structure including the ridge stripestructure. In conventional laser diodes having a buried heterostructure,formation such a current-confinement structure requires a number ofregrowth process steps. Thus, the fabrication process of the laser diodeis simplified in the present embodiment and the fabrication of the laserdiode is facilitated. Associated with this, the yield of production ofthe laser diode is improved.

[0512] Further, in view of the fact that the laser diode of FIG. 29 usesa wide ridge stripe structure having a width of 50 μm, and thus, a widecontact area is secured on the ridge stripe structure, it is possible todissipate heat efficiently via the contact area. Thereby, thedifferential resistance of the laser diode device is minimized.

[0513] Further, it should be noted that the optical waveguide layers2314 and 2135 and the active layer 2124 are free from Al in the laserdiode of the present embodiment. Referring back to FIG. 6 showing therelationship between the bandgap and the lattice constant for thecomposition of (Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P, which is widely usedin a visible laser diode of the AlGaInP system constructed on a GaAssubstrate, it can be seen that the same bandgap is realized by aGa_(0.7)In_(0.3)P composition that achieves a lattice matching with theGaP_(0.4)As_(0.6) substrate 2101. Thus, the present inventionsuccessfully uses the Ga_(0.7)In_(0.3)P composition for the opticalwaveguide layers 2134 and 2135 and minimizes the non-opticalrecombination of carriers, which is caused in relation to the existenceof Al. Thereby, the laser diode of the present embodiment can produce alarge output power. The present embodiment provides a red-wavelengthlaser diode operable under high temperature environment with a largeoutput optical power.

[0514] [Twenty-First Embodiment]

[0515]FIG. 30 shows the construction of a semiconductor optical deviceaccording to a twenty-first embodiment of the present invention, whereinthose parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0516] Referring to FIG. 30, the optical semiconductor device is a laserdiode and has a structure similar to that described with reference toFIG. 29, except that an etching stopper layer 2129 of GaInPAs having acomposition represented as Ga_(y)In_(1-y)P_(t)As_(1-t) (0<y≦1, 0≦t≦1) isinterposed between the to be-oxidized layer 2125 and the substrate 2121.In fact, the etching stopper layer 2129 is provided right underneath theto-be-oxidized layer 2125. It should be noted that a III-V materialhaving a high Al concentration or P concentration can be etchedeffectively by a hydrochloric acid etchant, while a material containingAs with high concentration resists against the etching process. Thus,the layer 2129 of the composition Ga_(y)In_(1-y)P_(t)As_(1-t) (0<y≦1,0≦t≦1) can be used as an etching stopper.

[0517] With the use of the etching stopper layer 2129, the etchingprocess for forming the ridge stripe structure is controlled easily, andthe yield of production of the laser diode is improved. Otherwise, thelaser diode of the present embodiment is similar to the laser diodedescribed with reference to FIG. 30.

[0518] [Twenty-Second Embodiment]

[0519]FIG. 31 shows the construction of a laser diode according to atwenty-second embodiment of the present invention.

[0520] Referring to FIG. 31, the laser diode is constructed on a GaAsoffset-substrate 2141 having an inclined principal surface inclined fromthe (100) surface in the [110] direction with an offset angle of 15°.

[0521] On the GaPAs substrate 2141, a cladding layer 2142 of n-typeAlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P (y=0.5, β=0.8) is formed by an MOCVDprocess with a thickness of 1 μm, and an optical waveguide layer 2154 ofAlGaInPAs having a composition represented as (Al_(x)Gal-z)_(γ)In_(1-γ)P(z=0.5, r=0.7) is formed on the cladding layer 2142 by an MOCVD processwith a thickness of 0.1 μm. Further, a quantum-well layer of AlGaInPAshaving a thickness of about 10 nm is formed on the optical waveguidelayer 2154 with a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P (x=0, α=0.65), wherein the composition ofthe quantum well layer is selected so as to accumulate a compressivestrain therein.

[0522] Further, an optical waveguide layer 2155 of p-type AlGaInPAshaving a composition represented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P(z=0.1, γ=0.7) is formed on the active layer 2124 with a thickness of0.1 μm, and a first p-type cladding layer 2143 of p-type AlGaInPAs isformed on the optical waveguide layer 2155 with a thickness of 0.1 μmand a composition represented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P (y=0.7,β=0.5).

[0523] On the first p-type cladding layer 2143, there is formed a layer2145 of p-type AlGaInPAs layer with a thickness of 50 nm such that thelayer 2145 has a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (x=1, y=0, t=0.037), wherein thiscomposition is actually represented as AlP_(0.037)As_(0.963).

[0524] Further, a second p-type cladding layer 2146 is formed on theAlPAs layer 2145 with a thickness of about 0.9 μm, wherein the p-typecladding layer 2146 has a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P (y=0.7, β=0.5). Further, a buffer layer2146 of p-type GaInP having a composition represented asGa_(0.5)In_(0.5)P and a contact layer 2157 of p-type GaAs are grownconsecutively on the second p-type cladding layer 2146 with respectivethicknesses of 0.1 μm and 0.2 nm.

[0525] In the foregoing layered structure, it should be noted that thecladding layers 2142, 2143 and 2146, the optical waveguide layers 2154and 2155, and the layer 2145 achieve a lattice matching with the GaAssubstrate 2141.

[0526] Next, the layered structure thus obtained is subjected to aphotolithographic patterning process to form a central ridge stripestructure, wherein the ridge stripe structure used in the embodiment ofFIG. 31 has a width of 50 μm. Thereby, the etching process of thephotolithographic patterning process is continued until the claddinglayer 2143 underneath the layer 2145 is exposed. Further, an oxidationprocess is conducted in a water vapor atmosphere at the temperature of450° C. to cause an oxidation in the AlP_(0.037)As_(0.963) layer 2145.Thereby, the oxidation starts at the exposed edge of the layer 2145 andproceeds to the interior of the ridge stripe structure along the layer2145, and a pair of oxidized regions 2147 are formed as a result suchthat each oxidized region 2147 extends into the interior of the ridgestripe structure from a side wall thereof along the layer 2145 with adistance of about 22.5 μm. Thereby, a region of unoxidized AlPAs layer2145 is left at the center of the two oxidized regions 2147 with a widthof 5 μm, wherein this unoxidized region provides the current path of thedrive current. On the other hand, the oxidized regions 2147 function asa current-blocking regions and there is formed a current-confinementstructure in the ridge stripe structure by the unoxidized part of theAlPAs layer 2145 and the oxidized regions 2147. In correspondence to theinjection of the drive current via the unoxidized part of the layer2145, there occurs a light emission right underneath the unoxidized partof the layer 2145. In the present embodiment, the ratio of the width ofthe unoxidized part to the entire width of the ridge stripe structure isabout 0.1.

[0527] After formation of the ridge stripe structure, the lateral sidesof the ridge stripe structure are filled with a polyimide as representedby regions 2148 and a p-type electrode 2158 is formed on the top part ofsuch a planarized structure in contact with the contact layer 2157.Further, the bottom surface of the GaAs substrate 2141 is polished to athickness of 100 μm, and an n-type electrode 2159 is formed on such apolished bottom surface.

[0528] In the embodiment of FIG. 31, too, a similar advantageous effectas the device describe previously is obtained. In the device of thepresent embodiment constructed on the GaAs substrate 2141, an adversaryeffect is expected when an AlAs layer is used for the to-be-oxidizedlayer 2145 due to the lattice misfit of as much as about 0.14%. Thepresent embodiment successfully avoids such an adversary effect by usingan AlGaInPAs layer containing P with the composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0<t≦1) for the layer 2145.By incorporating P into the layer 2145, it becomes possible to achieve alattice matching with the GaAs substrate 2141 and the adversary effectassociated with the strain in the layer 2145 is eliminated.

[0529] [Twenty-Third Embodiment]

[0530]FIG. 32 shows the construction of a semiconductor light-emittingdevice according to a twenty-third embodiment of the present invention,wherein those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0531] Referring to FIG. 32, the laser diode has a construction similarto that of the laser diode of FIG. 31 except that an AlAs layer 2165 ofp-type is provided in place of the AlGaInPAs layer 2145.

[0532] As a result of the selective oxidation of the AlAs layer 2165containing Al with high concentration, a part of the layer 2165 isconverted into insulator in correspondence to regions 2167, and theoxidized regions 2167 form the desired current-confinement structuretogether with the central unoxidized region of the AlAs layer 2165.

[0533] In view of the fact that the oxidized regions 2167 of the AlAslayer 2165 has a refractive index smaller than a refractive index of thelayer 2165 itself, there is formed a refractive index profile in thelayer 2165 and the refractive index profile forms a real-refractiveindex waveguide structure effective for lateral mode control. Forexample, it is possible to control the lateral mode of laser oscillationby optimizing the distance between the active layer 2144 and the layer2165 of AlAs.

[0534] As the waveguide structure is formed inside the ridge stripe witha sufficient distance from the side wall of the ridge stripe structure,the laser diode of the present embodiment successfully minimizes thewaveguide loss associated with the fluctuation of the edge width.

[0535] In the fabrication process of the laser diode of FIG. 32, itshould be noted only a single regrowth process is necessary for formingthe desired current-confinemen't structure including the ridge stripestructure. In conventional laser diodes having a buried heterostructure,formation such a current-confinement structure requires a number ofregrowth process steps. Thus, the fabrication process of the laser diodeis simplified in the present embodiment and the fabrication of the laserdiode is facilitated. Associated with this, the yield of production ofthe laser diode is improved.

[0536] Further, in view of the fact that the laser diode of FIG. 32 usesa wide ridge stripe structure having a width of 50 μm, and thus, a widecontact area is secured on the ridge stripe structure, it is possible todissipate heat efficiently via the contact area. Thereby, thedifferential resistance of the laser diode device is minimized.

[0537] Thus, the present embodiment enables a semiconductorlight-emitting device having a current-confinement structure and capableof lateral mode control by a simple fabrication process.

[0538] While the description has been provided so far with reference toa laser diode, the semiconductor light-emitting device of FIGS. 22-32may also be a light-emitting diode (LED). According to the presentinvention, a visible LED of high-luminosity and having an excellenttemperature characteristic can be obtained.

[0539] [Twenty-Fourth Embodiment]

[0540]FIG. 33 shows a construction of the layer 2125 used in a laserdiode according to a twenty-fourth embodiment of the present invention.

[0541] Referring to FIG. 33, the laser diode of the present embodimenthas a construction described already with reference to FIG. 29 or FIG.30, except that the layer 2125 is formed of an alternate stacking of anAlAs layer having a thickness of 5 nm and a layer having a latticeconstant between GaP and GaAs. In the illustrated example, the latterlayer is an AlPAs layer having a composition of AlP_(0.4)As_(0.6) and athickness of 1 nm, wherein the AlP_(0.4)As_(0.6) achieves a latticematching with the GaP_(0.4)As_(0.6) substrate 2121. By repeating theAlAs layer and the AlP_(0.4)As_(0.6) layer (four times in theillustrated example), there is formed a superlattice structure in thelayer 2125. While the AlAs layer has a lattice strain of about 1.4% withrespect to the AlP_(0.4)As_(0.6) substrate 2111, the AlAs layer can begrown on the substrate 2111 without lattice relaxation due to the smallthickness (5 nm).

[0542] With increasing thickness of the layer 2125, the oxidation rateof the layer 2125 increases. Further, the oxidation rate increases withincreasing Al content. Thereby, the lateral extent of the oxidizedregion 2127 is represented as being proportional to the square root ofthe duration of the oxidation process. Further, it turned out that theoxidation proceeds faster in the mixed crystal of AlPAs that contains Pthan in the mixed crystal of AlAs. Thus, it is preferable to use a mixedcrystal of AlAsP having a composition close to AlAs or AlAs itself forthe layer 2125 in order to reduce the duration for the oxidationprocess.

[0543] In the case the GaP_(0.4)As_(0.6) substrate 2121 is used for thesubstrate of the laser diode, it should be noted that the AlAs layeraccumulates a lattice strain of 1.4%. Thus, it is necessary to limit thethickness of the AlAs layer to be smaller than a critical thicknessabove which lattice relaxation takes place. On the other hand, such arestriction of thickness of the AlAs layer decreases the oxidation rate.On the other hand, the construction of FIG. 33, in which a number ofAlAs layers, each having a thickness smaller than the critical thicknessof the AlAs layer, are stacked repeatedly and alternately with anintervening layer, is effective for preventing lattice relaxation andfor realizing a sufficient oxidation rate.

[0544]FIG. 34 shows the result of an experiment conducted by theinventor of the present invention.

[0545] In the experiment, the structure of FIG. 33 is used and a layeridentical in composition with the p-type cladding layer 2123 is grownthereon with a thickness of 0.2 μm. Next, the cladding layer 2123 thusformed is patterned until the etching stopper layer 2129 (see FIG. 30)is exposed, and a ridge stripe structure is formed with a width of 40μm.

[0546] The structure thus formed is subjected to a selective oxidationprocess at 460° C. for 10 minutes.

[0547]FIG. 34 shows the plan view of the specimen used in the experimentwherein FIG. 34 shows the ridge region and the region of the layer 2125where the selective oxidation has taken place. It should be noted thatthe region where the selective oxidation has taken place is representedin FIG. 34 by hatching. As can be seen in FIG. 34, the oxidized regionis formed with a lateral width of 8 μm only after 10 minutes ofselective oxidation process. This rate of oxidation is sufficient forpractical use of the selective oxidation process for the formation ofthe oxidized regions 2127 in the actual fabrication process of the laserdiode. This rapid oxidation is attributed to the large diffusion rate ofoxygen taking place along the surface of the layer 2125. By using thestructure of FIG. 33, the number of the surfaces available for oxygendiffusion is increased, and this leads to the increase of the totaloxidation rate of the layer 2125.

[0548] It should be noted that the superlattice structure of FIG. 33 isapplicable to any of the embodiments from FIGS. 22-32. Further, itshould be noted that the AlP_(0.4)As_(0.6) layer in the construction ofFIG. 33 may be replaced with any material of the system GaAsP, AlInP,GaInP, AlGaInP, GaInAsP, and AlGaInAsP, provided that the material has alattice constant that eliminates lattice relaxation by the AlAs layer.The layer may achieve a lattice matching with the substrate oraccumulate a strain compensating the strain of the AlAs layer. In viewof the rapid rate of oxidation, the material of AlPAs, which contains Alas the only group III element, is most preferable. The thickness of thelayers constituting the superlattice structure of FIG. 33 may be changedvariously from the value described before.

[0549] [Twenty-Fifth Embodiment]

[0550]FIG. 35 shows the construction of a vertical-cavity laser diodeaccording to a twenty-fifth embodiment of the present invention.

[0551] It should be noted that the laser diode of the present inventionuses a distributed Bragg reflector (DBR) having a lattice constantbetween GaAs and GaP, wherein at least one of the two semiconductorlayers repeated alternately to form the distributed Bragg reflector, hasa composition represented as(Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0.5≦y₁≦1,0<z₁<1).

[0552] In the system of AlGaInAsP, it should be noted that the bandgapenergy is increased with decrease of the lattice constant. See therelationship of FIG. 8. Thus, the DBR based on the semiconductor layersof the AlGaInAsP system and having a lattice constant between GaAs andGaP does not cause absorption of the optical radiation emitted by thelaser diode with the wavelength of 630-650 nm. Thereby, the opticalwaveguide loss caused by the DBR is minimized.

[0553] Further, in view of the fact that the semiconductor layercontains As, the hillock density or surface defects including surfaceundulation of the semiconductor layers constituting the DBR is reduced.Thereby, the reflectance of the DBR is maximized.

[0554] It is known that, in the semiconductor mixed crystal such asAlInP or AlGaInP, there is a tendency of increasing hillock density andsurface undulation with increasing Al content. While this problem can bereduced, to some extent, by using an offset substrate having a surfaceoffset from the (100) surface or increasing the growth temperature, ithas been difficult to suppress the hillock formation or surfaceundulation perfectly.

[0555] The present inventor discovered experimentally that hillockformation is effectively suppressed by adding As into the mixed crystalof AlGaInP. Thereby, it was also discovered that only a small amount ofAs, such as 1-2% in terms of the atomic fraction for the group Velements, is sufficient for achieving the desired effect. In achievingthe desired effect, it is not necessary to restrict the growth conditionor surface orientation of the substrate. Thus, by using a mixed crystalof the AlGaInAsP system containing As for the DBR, it becomes possibleto improve the quality of the surface of the crystal layers forming theDBR.

[0556]FIG. 35 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0557] Referring to FIG. 35, the vertical-cavity laser diode isconstructed on a substrate 3102 of n-type GaAsP having a lattice misfitof −1.4% with respect to a GaAs substrate and includes, on the substrateof 3102, a buffer layer 3103 of n-type GaAsP, a DBR structure 3104formed of an alternate repetition of an n-type AlInAsP layer and ann-type GaInAsP layer, a cladding layer 3105 of undoped AlGaInAsP, anactive layer 3106 of undoped GaInP, a cladding layer 3107 of undopedAlGaInAsP, a DBR structure 3108 formed of an alternate repetition of ap-type AlInAsP layer and a p-type GaInAsP layer, a spike eliminationlayer 3109 of GaInP, and a contact layer 3110 of GaAsP, wherein thelayers 3103-3110 are deposited consecutively on the substrate 3102 by anMOCVD process.

[0558] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBR 3108,the spike elimination layer 3109 and the contact layer 3110 arepatterned to form a central post structure, wherein the patterningprocess is conducted until the AlGaInAsP cladding layer 3107 is exposed.In the construction of FIG. 35, it should be noted that the layers3103-3110 achieve a lattice matching with the GaAsP substrate 3102.

[0559] After the formation of the central post structure, an SiO₂ film3111 is deposited uniformly by a CVD process so as to cover the centralpost structure, and a photolithographic patterning process is conductedto form a first contact window in the SiO₂ film 3111 by using a resistmask such that the first contact window exposes the GaAsP contact layer3110 at top part of the central post structure. Further, the contactlayer 3110 is patterned by using another photolithographic process so asto expose the spike elimination layer 3109 in correspondence to a secondcontact window formed in the first contact window, and a circular resistmask pattern is formed so as to cover the spike elimination layer 3109thus exposed such that the circular resist mask pattern is locatedcentrally to the spike elimination layer 3909 exposed in the secondcontact window.

[0560] Further a p-type electrode layer is deposited on the structurethus covered by the circular resist mask by an evaporation-depositionprocess, and a p-type electrode 3112 is formed by lifting off thecircular resist mask. Further, the bottom surface of the GaAsP substrate3102 is polished and an n-type electrode 3101 is deposited by anevaporation-deposition process.

[0561] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3101 and 3112.

[0562] In the laser diode of the present embodiment, the laser beam isemitted from the circular opening formed in the p-type electrode 3112.In order to facilitate the emission of the laser beam, the GaAsP contactlayer 3110, which is not transparent to the laser beam, is removed incorrespondence to the second contact window.

[0563] As is well known in the art, each of the layers constituting theDBR structure 3104 or 3108 has a thickness set to be equal to a quarterwavelength of the laser beam produced by the laser diode. Further, thecladding structure including the cladding layers 3105 and 3107 and theactive layer 3106 is set to be equal to an integer multiple of thehalf-wavelength optical distance. In the case the refractive index ofthe semiconductor layers constituting the DBR structure adjacent to thecladding layer is smaller than the refractive index of the claddinglayer, a full-wavelength optical cavity is formed. In the opposite case,a half-wavelength optical cavity is formed.

[0564] According to the present embodiment, the active layer 3106 has acomposition of GaInP causing a laser oscillation at the wavelength of635 nm, wherein the active layer 3106 of such a composition is appliedwith a compressive strain from the substrate 3102. In view of the factthat the DBR structure includes, at least a part thereof, a layer ofAlGaInAsP or AlInAsP that contains As with a concentration of 2% withrespect to the group V elements. Thus, the hillock formation iseliminated on the surface of the semiconductor layers constituting theDBR structure and a uniform inter interface is realized. Associatedtherewith, the characteristic of the DBR structure is improved and theperformance of the laser diode is improved with respect to theoscillation threshold current and device lifetime.

[0565] [Twenty-Sixth Embodiment]

[0566] Next, a vertical-cavity laser diode according to a twenty-sixthembodiment of the present invention will be described with reference toFIG. 36.

[0567] In the laser diode of the present embodiment, a DBR having acomposition between GaAs and GaP is used similarly to the laser diode ofFIG. 35, except that the active layer is formed of GaInAsP. Morespecifically, the laser diode of the present embodiment uses acomposition of Ga_(y2)In_(1-y2)As_(z2)P_(1-z2) (0<y₂≦1, 0<z₂≦1) for theactive layer, in combination with the DBR having a lattice constantbetween GaAs and GaP.

[0568] According to the present embodiment, it is possible to controlthe oscillation wavelength and further the strain of the active layerwith respect to the DBR structure by controlling the As content in theactive layer of GaInAsP.

[0569] It should be noted that the wavelength obtained from a mixedcrystal of GaInP having a lattice matching composition to a GaAssubstrate is about 650 nm, wherein this wavelength decreases withdecrease of the lattice constant of the GaInP mixed crystal. Thus, it isnecessary to increase the Ga content in such a GaInP active layer forincreasing the wavelength, while such an increase of the Ga contentcauses accumulation of a compressive strain in the active layer.

[0570] Meanwhile, it is possible, in a GaInAsP active layer to decreasethe bandgap energy by increasing the As content. While such an increaseof As content induces an increase of the lattice constant, the increaseof the lattice constant can be successfully compensated for by using aGaInP composition having a small lattice constant as the startingcomposition of the active layer and add As to such a startingcomposition. As the change of the bandgap energy induced by As is muchlarger than the change of the bandgap energy caused by the associatedlattice strain or a change of Ga content in a GaInP mixed crystal, theforegoing construction of the present embodiment easily increases theoscillation wavelength and achieves minimization of the lattice misfitwith respect to the DBR.

[0571] For example, it is necessary to use a composition ofGa_(0.45)In_(0.55)P for obtaining an oscillation wavelength of 660 nmwhen a GaInP layer formed on a Ga_(0.7)In_(0.3)P substrate is used forthe active layer. In this case, a strain of about 1.9% is accumulated inthe GaInP active layer. In the case of the present embodiment, in whicha composition of Ga_(0.8)In_(0.2)As_(0.5)P_(0.5) is used for the activelayer, it is possible to achieve a laser oscillation at the wavelengthof 660 nm while reducing the strain to one half (½).

[0572] Further, the use of the mixed crystal of GaInAsP for the activelayer reduces the problem of deterioration of crystal quality. Thus, thepresent embodiment enables the desired oscillation wavelength whilereducing the strain in the active layer as compared with the case ofachieving the foregoing desired oscillation wavelength while using aGaInP mixed crystal for the active layer.

[0573] Further, the present embodiment has an advantageous feature inthat the lattice constant of the DBR can be set close to the latticeconstant of GaP. Thereby, the refractive index difference between theAlInAsP layer and the GaInAsP layer constituting the DBR structure isincreased and the number of stacks of the layers in the DBR structurecan be reduced.

[0574]FIG. 36 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0575] Referring to FIG. 36, the vertical-cavity laser diode isconstructed on a substrate 3202 of n-type GaAsP having a lattice misfitof −2.0% with respect to a GaAs substrate and includes, on the substrateof 3202, a buffer layer 3203 of n-type GaAsP, a DBR structure 3204formed of an alternate repetition of an n-type AlInAsP layer and ann-type GaAsP layer, a cladding layer 3205 of undoped AlGaInAsP, anactive layer 3206 of undoped GaInAsP, a cladding layer 3207 of undopedAlGaInAsP, a DBR structure 3208 formed of an alternate repetition of ap-type AlInAsP layer and a p-type GaAsP layer, a spike elimination layer3209 of p-type GaInP, and a contact layer 3210 of p-type GaAsP, whereinthe layers 3203-3210 are deposited consecutively on the substrate 3202by an MOCVD process.

[0576] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBRstructure 3208, the spike elimination layer 3209 and the contact layer3210 are patterned to form a central post structure. The patterningprocess is conducted until the AlGaInAsP cladding layer 3207 is exposed.

[0577] In the construction of FIG. 36, it should be noted that the GaAsPlayer forming the DBR structures 3204 and 3208 achieves a latticematching with the GaAsP substrate 3202. It should be noted that theGaAsP layer having such a lattice matching composition to the GaAsPsubstrate 3202 is transparent to the optical radiation produced by thelaser diode.

[0578] After the formation of the central post structure, an SiO₂ film3211 is deposited uniformly on the central post structure by a CVDprocess, and a photolithographic patterning process is conducted to forma first contact window in the SiO₂ film 3211 by using a resist mask soas to expose the GaAsP contact layer 3210 at top part of the centralpost structure. Further, the contact layer 3210 is patterned by usinganother photolithographic process as to expose the spike eliminationlayer 3209 in a second contact window formed in the first contactwindow, and a circular resist mask pattern is formed so as to cover thespike elimination layer 3209 thus exposed by the second contact window.The circular resist mask is formed centrally to the second contactwindow.

[0579] Further a p-type electrode layer is deposited on the structurethus covered by the circular resist mask by an evaporation-depositionprocess, and a p-type electrode 3212 is formed by lifting off thecircular resist mask. Further, the bottom surface of the GaAsP substrate3202 is polished and an n-type electrode 3201 is deposited by anevaporation-deposition process.

[0580] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3201 and 3212.

[0581] In the laser diode of the present embodiment, the laser beam isemitted from the circular opening formed in the p-type electrode 3212.In order to facilitate the emission of the laser beam, the GaAsP contactlayer 3210, which is not transparent to the laser beam, is removed incorrespondence to the second contact window.

[0582] In the laser diode of FIG. 36, it should be noted that the activelayer 3206 may have the foregoing composition ofGa_(0.8)In_(0.2)As_(0.5)P_(0.5). Further, the DBR structure 3204 isformed of an alternate stacking of an n-type AlInAsP layer and an n-typeGaAsP layer. The DBR structure 3208, on the other hand, is formed of analternate stacking of a p-type AlInAsP layer and a p-type GaAsP layer.In the illustrated example, a composition that achieves a lattice misfitof −2.0% with respect to a GaAs substrate is used for the GaAsPsubstrate 3202 as noted already.

[0583] Similarly to the previous embodiment, the cladding layers 3205and 3207 use a composition of AlGaInAsP that contains As. Further, theDBR structures 3204 and 3208 use the alternate stacking of the layers ofAlInAsP and GaAsP that contains As therein. Thus, the hillock formationat the semiconductor layer interface in the DBR structure is effectivelysuppressed. It should be noted that the layer of GaAsP used in the DBRstructure 3204 or 3208 is transparent to the laser oscillationwavelength in the composition that achieves lattice matching with theGaAsP substrate 3202.

[0584] By using a composition providing an oscillation wavelength of 650nm for the active layer 3206, it is possible to reduce the latticestrain of the active layer to be one half (½). Further, it is possibleto set the lattice constant of the GaAsP substrate 3202 to be close tothe lattice constant of GaP. Thus, a large refractive index differenceis achieved between the semiconductor layers constituting the DBRstructures 3204 and 3208, and the number of stacks in the DBR structurecan be reduced.

[0585] Because of the reduced strain, the quality of the crystalconstituting the active layer 3206 is improved. As a result of decreaseof the number of stacks of the semiconductor layers in the DBRstructures, the resistance of the laser diode is also reduced.

[0586] [Twenty-Seventh Embodiment]

[0587] Next, a vertical-cavity laser diode according to a twenty-seventhembodiment of the present invention will be described with reference toFIG. 37.

[0588] In the laser diode of the present embodiment, the laser diodeincludes a DBR structure having a lattice constant between GaP and GaAs,and a pair of carrier confinement layers having a compositionrepresented as Ga_(y3)In_(1-y3)P (0.5<y₃≦1) are provided so as tosandwich the active layer 3206 vertically.

[0589] As can be seen in FIG. 8, the bandgap energy increases in thematerial of the system GaInP with decreasing lattice constant. Thus, theGaInP layer having a lattice matching composition with the DBR structurehas a bandgap energy larger than the optical wavelength range of 630-650nm and functions as an effective carrier confinement layer with regardto the active layer 3306.

[0590] According to the present embodiment, carrier confinement isachieved by a semiconductor layer of GaInP, which is free from Al. Thus,the problem of non-optical recombination of carriers associated with theuse of an Al-containing layer such as an AlGaInP layer is successfullyavoided. The laser diode of the present embodiment has an advantageousfeature of low threshold of laser oscillation.

[0591] Further, the vertical-cavity laser diode of the presentembodiment uses a semiconductor layer transparent to the opticalradiation of the wavelength of laser oscillation for the contact layer.As a result of use of such a transparent contact layer, it becomespossible to eliminate the patterning process to remove the contact layer3110 or 3210 in the previous embodiment for forming the optical window.

[0592]FIG. 37 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0593] Referring to FIG. 37, the vertical-cavity laser diode isconstructed on a substrate 3302 of n-type GaAsP and includes, on thesubstrate of 3302, a buffer layer 3303 of n-type GaAsP, a DBR structure3304 formed of an alternate repetition of an n-type AlInAsP layer and ann-type GaInP layer, a carrier confinement layer 3305 of undoped GaInP,an active layer 3306 of undoped GaInAsP, another carrier confinementlayer 3307 of undoped GaInP, another DBR structure 3308 formed of analternate repetition of a p-type AlInAsP layer and a p-type GaInP layer,and a contact layer 3309 of p-type GaInP, wherein the layers 3303-3309are deposited consecutively on the substrate 3302 by an MOCVD process.

[0594] After the formation of the foregoing layered structure, aphotolithographic-patterning process is conducted in which the DBRstructure 3308, and the contact layer 3309 are patterned to form acentral post structure. The patterning process is conducted until theGaInP optical waveguide layer 3307 is exposed.

[0595] After the formation of the central post structure, an SiO₂ film3310 is deposited uniformly by a CVD process, and a photolithographicpatterning process is conducted to form a contact window in the SiO₂film 3310 so as to expose the GaInP contact layer 3309 in correspondenceto the contact window at top part of the central post structure.Further, a circular resist mask pattern is formed so as to cover thecontact layer 3309 thus exposed by the contact window, and a p-typeelectrode layer is deposited on the structure thus covered by thecircular resist mask by an evaporation-deposition process. By liftingoff the circular resist pattern, a p-type electrode 3311 is formed in acircular shape. Further, the bottom surface of the GaAsP substrate 3302is polished and an n-type electrode 3301 is deposited by anevaporation-deposition process.

[0596] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3301 and 3311.

[0597] In the laser diode of the present embodiment, the laser beam isemitted from the circular opening formed in the p-type electrode 3311.Because the GaInP contact layer 3309 is transparent to the optical beamproduced by the laser diode, the process for forming an optical windowin the contact layer 3309 as in the case of the previous embodiments ofFIGS. 35 and 36 can be eliminated.

[0598] It should be noted that the GaInP mixed crystal having a latticeconstant between GaAs and GaP is transparent to the optical radiation inthe wavelength range of 630-660 nm. Thus, the GaInP contact layer 3309can be formed with lattice matching to the GaAs substrate 3302. Further,it is possible to use a GaAsP layer for the transparent contact layer3309, provided that the GaAsP layer has an As content smaller than about0.63. A GaAsP layer containing As with a concentration exceeding theforegoing limit shows an optical absorption to the optical beam producedby the laser diode. It should be noted that a GaAsP mixed crystal layerhaving such a composition can achieve a lattice matching with the GaAsPsubstrate 3302. In the case of using a GaAsP layer for the contact layer3309, a high-concentration doping can be achieved easily. In the event atransparent GaAsP layer cannot be obtained at the lattice matchingcomposition to the DBR structure or the substrate, it is possible to usea transparent GaAsP layer accumulating a strain.

[0599] As noted previously, the present embodiment can eliminate theproblem of non-recombination of carriers as a result of use of Al-freecomposition for the layers 3304 and 3307 and the efficiency of laseroscillation is improved substantially.

[0600] [Twenty-Eighth Embodiment]

[0601] Next, a vertical-cavity laser diode according to a twenty-eighthembodiment of the present invention will be described with reference toFIG. 38.

[0602] In the laser diode of the present embodiment that uses a DBRstructure having a lattice constant between the lattice constant of GaAsand the lattice constant of GaP, an AlAsP layer having a compositionrepresented as AlAs_(z4)P_(1-z4) (0≦z₄≦1) is used for constructing theDBR structure. By using the AlAsP layer, it is possible to increase thereflectance of the DBR structure. Thereby, the number of stacks of thesemiconductor layers in the DBR structure can be reduced. In the DBRstructure having a lattice constant between GaAs and GaP, it is possibleto use a mixed crystal layer of AlAsP in addition to AlInP.

[0603] It is estimated that a mixed crystal of AlAsP has a smallerrefractive index as compared with a mixed crystal of AlInP of the samelattice constant, due to the reason that the mixed crystal of AlAsP hasa larger bandgap energy between the r point of the conduction band andthe valence band. As the number of stacks of layers in the DBR structureis reduced by using the staking structure of AlAsP/GaInP as comparedwith the case of using the stacking structure of AlInP/GaInP, it ispossible to achieve a high reflectance with a reduced number of thestacks and the resistance of the laser diode caused by the DBR structureis reduced. In view of the fact that the AlAsP mixed crystal is freefrom In, the relative proportion of As in the mixed crystal is increasedand the problem of hillock formation is effectively suppressed.

[0604]FIG. 38 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0605] Referring to FIG. 38, the vertical-cavity laser diode isconstructed on a substrate 3402 of n-type GaAsP and includes, on thesubstrate of 3402, a buffer layer 3403 of n-type GaAsP, a DBR structure3404 formed of an alternate repetition of an n-type AlAsP layer and ann-type GaInP layer, a carrier confinement layer 3405 of undoped GaInP,an active layer 3406 of undoped GaInAsP, another carrier confinementlayer 3407 of undoped GaInP, another DBR structure 3408 formed of analternate repetition of a p-type AlAsP layer and a p-type GaInP layer,and a contact layer 3409 of p-type GaInP, wherein the layers 3403-3409are deposited consecutively on the substrate 3402 by an MOCVD process.

[0606] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBRstructure 3408 and the contact layer 3409 are patterned to form acentral post structure. The patterning process is conducted until theGaInP optical waveguide layer 3407 is exposed.

[0607] After the formation of the central post structure, an SiO₂ film3410 is deposited uniformly by a CVD process, and a photolithographicpatterning process is conducted to form a contact window in the SiO₂film 3410 so as to expose the GaInP contact layer 3409 in correspondenceto the contact window at top part of the central post structure.Further, a circular resist mask pattern is formed so as to cover thecontact layer 3409 thus exposed by the contact window, and a p-typeelectrode layer is deposited on the structure thus covered by thecircular resist mask by an evaporation-deposition process. By liftingoff the circular resist pattern, a p-type electrode 3411 is formed in acircular shape. Further, the bottom surface of the GaAsP substrate 3402is polished and an n-type electrode 3401 is deposited by anevaporation-deposition process.

[0608] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3401 and 3411.

[0609] In the vertical-cavity laser diode of FIG. 38, the AlAsP layerand the GaInP layer constituting the DBR structure 3404 or 3408 have alattice matching composition to the GaAsP substrate 3402. As the AlAsPlayer has a smaller refractive index as compared with the AlInP layer ofthe same lattice constant, it is possible to increase the refractiveindex difference or step formed between the AlAsP layer and the GaInPlayer in the DBR structure 3404 or 3408. As a result, the number ofstacks of layers in the DBR structures 3404 and 3408 is reduced and theresistance of the laser diode is reduced accordingly. In the DBRstructure 3404 or 3408, it should be noted that the AlAsP layer may beused together with a semiconductor layer other than GaInP. For example,the AlAsP layer may be used together with a layer of AlGaAsP to form theDBR structure. In this case, the superlattice structure of the DBRstructure 3404 or 3408 is easily formed by an MOCVD process while merelyswitching the supply of gaseous source of Ga.

[0610] [Twenty-Ninth Embodiment]

[0611] Next, a vertical-cavity laser diode according to a twenty-ninthembodiment of the present invention will be described with reference toFIG. 39.

[0612] In the present embodiment, the laser diode includes acurrent-confinement structure formed in a part of the DBR structure,wherein the current-confinement structure is formed in the DBR structureby a selective oxidation process of an AlAsP layer having a compositionrepresented as AlAs_(z5)P_(1-z5) (0≦z₅≦1). The AlAsP layer has a lowrefractive and forms the DBR structure together with anothersemiconductor layer of a high refractive index.

[0613] It should be noted that the foregoing AlAsP layer is notnecessarily be the only one low-refractive-index layer of the DBRstructure. For example, the AlAsP layer may be formed only in thevicinity of the active layer. In this case, the low-refractive-indexlayer in the region away from the active layer may be formed of AlInAsP.By doing so, the current-confinement structure can be formed withoutincreasing the resistance of the laser diode.

[0614] The AlAsP layer is not required to achieve a lattice matchingwith other layers of the DBR structure but may accumulate a straintherein. As the AlAsP layer is used only in a part of the DBR structure,there occurs no serious degradation of crystal quality even when theAlAsP layer accumulates a strain.

[0615] According to the present embodiment, a vertical-cavity laserdiode having a reduced threshold current is obtained. Because of the useof AlAsP for the part of the DBR structure where the selective oxidationprocess is to be conducted, the selective oxidation process proceedsrapidly. It should be noted that the layer of AlAsP contains Al as theonly group III element. It should be noted that the oxide layer formedas a result of the oxidation of Al becomes an insulator. Thereby, drivecurrent of the laser diode is caused to flow through the unoxidizedregion encircled by the insulating region thus oxidized.

[0616]FIG. 39 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0617] Referring to FIG. 39, the vertical-cavity laser diode isconstructed on a substrate 3502 of n-type GaAsP and includes, on thesubstrate of 3502, a buffer layer 3503 of n-type GaAsP, a first DBRstructure 3504 formed of an alternate repetition of an n-type AlAsPlayer and an n-type GaInP layer, a carrier confinement layer 3505 ofundoped GaInP, an active layer 3506 of undoped GaInAsP, another carrierconfinement layer 3507 of undoped GaInP, a second DBR structure 3508formed of an alternate repetition of a p-type AlAsP layer and a p-typeGaInP layer, a third DBR structure 3509 formed of an alternaterepetition of a p-type AlInAsP layer and a p-type GaInP layer, and acontact layer 3510 of p-type GaInP, wherein the layers 3403-3510 aredeposited consecutively on the substrate 3502 by an MOCVD process.

[0618] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBRstructure 3508, 3509 and the contact layer 3510 are patterned to form acentral post structure. The patterning process is conducted until theGaInP optical waveguide layer 3507 is exposed.

[0619] After the formation of the central post structure, a selectiveoxidation process is conducted in a water vapor atmosphere to induce aselective oxidation of the AlAsP layer constituting the second DBRstructure 3508. The oxidation of the AlAsP layer proceeds laterally intothe interior of the central post structure in the DBR structure 3508along the AlAsP layers therein, and there is formed an oxidized region3508A such that the oxidized region 3508A surrounds the central,unoxidized region that provides the current path of the drive current.

[0620] Further, an SiO₂ film 3511 is deposited uniformly by a CVDprocess, and a photolithographic patterning process is conducted to forma contact window in the SiO₂ film 3511 so as to expose the GaInP contactlayer 3510 in correspondence to the contact window at top part of thecentral post structure. Further, a circular resist mask pattern isformed so as to cover the contact layer 3510 thus exposed by the contactwindow, and a p-type electrode layer is deposited on the structure thuscovered by the circular resist mask by an evaporation-depositionprocess. By lifting off the circular resist pattern, a p-type electrode3512 is formed in a circular shape. Further, the bottom surface of theGaAsP substrate 3502 is polished and an n-type electrode 3501 isdeposited by an evaporation-deposition process.

[0621] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3501 and 3512.

[0622] In the present embodiment, the process of selective oxidation forforming the oxidized region 3508A is substantially facilitated byforming the second DBR structure 3508 by a repetitive and alternatestacking of AlAsP and GaInP layers. While it is possible to form thethird DBR structure 3509 also to have the AlAsP/GaInP structuresimilarly to the second DBR structure, it is advantageous to use theAlInAsP/GaInP stacking structure for the third DBR structure 3509 forminimizing the resistance of the laser diode.

[0623] Further, it is possible to use a stacking structure of AlAs/GaInPfor the second DBR structure 3508.

[0624] According to the present embodiment, a current confinementstructure is formed inside the DBR structure and the threshold currentof laser oscillation can be reduced substantially.

[0625] [Thirtieth Embodiment]

[0626] Next, a vertical-cavity laser diode according to a thirtiethembodiment of the present invention will be described with reference toFIG. 40.

[0627] In the present embodiment, the vertical-cavity laser diodeincludes a DBR structure similarly to the vertical-cavity laser diodesof the previous embodiments except that there is interposed a currentconfinement structure of an AlAsP layer between the DBR structure andthe active layer, wherein the AlAsP layer has a composition representedas AlAs_(z6)P_(1-z6) (0≦z₆≦1) and includes therein an insulator regionformed as a result of selective oxidation.

[0628] In the present embodiment, it is not necessary for the AlAsPlayer to achieve a lattice matching to the DBR structure but mayaccumulate a strain. Because a sufficient current-confinement effect isobtained with the thickness of only 10-20 nm for the AlAsP layer, it isalso possible to use an AlAs layer in place of the AlAsP layer withoutcausing any serious deterioration of crystal quality.

[0629] In the laser diode of the present embodiment, it is preferable toprovide the current-confinement structure of AlAsP as close to theactive layer as possible for eliminating unwanted spreading of the drivecurrent after passing through the current-confinement structure. Theoxidized region thus formed as a result of the selective oxidationprocess has a reduced refractive index, and the current-confinementstructure forms also an optical confinement structure, which iseffective for lateral mode control of the laser oscillation.

[0630]FIG. 40 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0631] Referring to FIG. 40, the vertical-cavity laser diode isconstructed on a substrate 3602 of n-type GaAsP and includes, on thesubstrate of 3602, a buffer layer 3603 of n-type GaAsP, a DBR structure3604 formed of an alternate repetition of an n-type AlAsP layer and ann-type GaInP layer, a first carrier confinement layer 3605 of undopedGaInP, an active layer 3606 of undoped GaInAsP, a second carrierconfinement layer 3607 of undoped GaInP, a to-be-oxidized layer 3608 ofp-type AlAsP, a third optical confinement layer 3609 of undoped GaInP, acontact layer 3610 of p-type GaInP, and another DBR structure 3611,wherein the layers 3603-3611 are deposited consecutively on thesubstrate 3602 by an MOCVD process.

[0632] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBRstructure 3611 is patterned to form a central post structure. Thepatterning process is conducted until the GaInP contact layer 3610 isexposed.

[0633] After the formation of the central post structure, the centralpost structure is protected by a circular resist pattern, and the GaInPcontact layer 3610, the GaInP carrier confinement layer 3609 and theAlAsP to-be-oxidized layer 3608 are patterned consecutively until thecarrier confinement layer 3607 is exposed, while using the circularresist pattern as a mask.

[0634] Next, the structure thus obtained is subjected to a selectiveoxidation process in a water vapor atmosphere, and there is formed anoxidized region 3608A in the to-be-oxidized layer 3608 as a result ofthe oxidation that proceeds toward the interior of the layer 3608,starting from the outermost, exposed surface. Thereby, the oxidizedregion 3608A acts as a current-blocking region and there is formed acurrent-confinement structure within the to-be-oxidized layer 3608.

[0635] Further, an SiO₂ film 3612 is deposited uniformly by a CVDprocess, and a photolithographic patterning process is conducted to forma contact window in the SiO₂ film 3612 so as to expose DBR structure3611 and a part of the p-type GaInP contact layer 3610. Further, aresist mask pattern is formed so as to cover the DBR structure 3611 anda p-type electrode layer is deposited on the structure thus covered bythe resist mask pattern by an evaporation-deposition process. By liftingoff the resist mask pattern, a p-type electrode 3613 is formed incontact with the contact layer 3610. Further, the bottom surface of theGaAsP substrate 3602 is polished and an n-type electrode 3601 isdeposited by an evaporation-deposition process.

[0636] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3601 and 3613.

[0637] As noted before, the vertical-cavity laser diode of FIG. 40includes a current-confinement structure formed as a result of selectiveoxidation process of the AlGaP to-be-oxidized layer 3608. As the layer3608 is formed close to the active layer 3606, the carrierscorresponding to the drive current of the laser diode are injected intothe active layer 3606 with minimum lateral spreading, and the efficiencyof laser oscillation is improved substantially. Further, the oxidizedregion 3608A and the unoxidized region of the layer 3608 form an opticalconfinement structure effective for lateral mode control of laseroscillation. Thereby, the vertical-cavity laser diode of the presentembodiment oscillates stably at a single lateral mode.

[0638] It should be noted that a thickness of 10-20 nm is sufficient forthe AlAsP to-be-oxidized layer 3608. Further, AlAs may be used for thelayer 3608 without causing a serious deterioration of crystal quality.

[0639] It should be noted that the foregoing embodiments of FIGS. 35-40can be constructed also on a GaInP substrate. Such a GaInP substrate canbe formed by depositing a composition-graded layer on a GaAs substrateby a vapor phase epitaxial process.

[0640] [Thirty-First Embodiment]

[0641] Next, a vertical-cavity laser diode according to a thirty-firstembodiment of the present invention will be described with reference toFIG. 31.

[0642] In the present embodiment, the vertical-cavity laser diode uses apair of DBR structures having a lattice constant between GaAs and GaP,wherein the vertical-cavity laser diode is constructed such that anoutput laser beam is obtained through the DBR structure located closerto the substrate. The vertical-cavity laser diode of such a constructionis suitable for a flip-chip mounting, as the laser beam is emitted inthe upward direction in the state that the laser diode is mounted on asupport substrate such as a printed circuit board in a face-down stateor junction-down state.

[0643]FIG. 41 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0644] Referring to FIG. 41, the vertical-cavity laser diode isconstructed on a substrate 3813 of n-type GaP carrying thereon acomposition-graded layer 3814 of n-type GaAsP formed by a vapor phaseepitaxial process, wherein the composition-graded layer 3814 changes acomposition thereof from GaP to GaAsP.

[0645] The laser diode includes, on the composition-graded layer 3814, abuffer layer 3803 of n-type GaAsP, a DBR structure 3804 formed of analternate repetition of an n-type AlInAsP layer and an n-type GaInPlayer, a carrier confinement layer 3805 of undoped AlGaInAsP, an activelayer 3806 of undoped GaInAsP, another carrier confinement layer 3807 ofundoped GaInP, a DBR structure 3808 formed of an alternate repetition ofa p-type AlInAsP layer and a p-type GaInP layer, a spike eliminationlayer 3809 of p-type GaInP, and a contact layer 3810 of p-type GaAsP,wherein the layers 3803-3810 are deposited consecutively on thecomposition-graded layer 3814 by an MOCVD process.

[0646] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBR 3808,the spike elimination layer 3809 and the contact layer 3810 arepatterned to form a central post structure while using a resist mask.The patterning process is conducted until the GaInP carrier confinementlayer 3807 is exposed.

[0647] After the formation of the central post structure, an SiO₂ film3811 is deposited uniformly by a CVD process, and a photolithographicpatterning process is conducted to form a contact window in the SiO₂film 3811 by using a resist mask so as to expose the GaAsP contact layer3810, and a p-type electrode 3812 is deposited by anevaporation-deposition process.

[0648] In the present embodiment, the bottom surface of the GaAsPsubstrate 3802 is polished and a resist pattern is provided so as tocover the region aligned with the post structure, and an n-typeelectrode is deposited by an evaporation-deposition process. Further, bylifting off the resist pattern, there is formed an n-type electrode 3801such that the n-type electrode 3801 has an optical window incorrespondence to the part where the resist pattern has been provided.

[0649] Thereafter, a thermal annealing process is applied to form anohmic contact at each of the electrodes 3801 and 3812.

[0650] Finally, an SiO₂ anti-reflection coating 3815 is provided on thebottom surface of the substrate 3801 in correspondence to the opticalwindow with a thickness corresponding to a quarter wavelength of thelaser oscillation wavelength.

[0651] In the construction of FIG. 41, it should be noted that the GaPsubstrate 3813 and the GaAsP composition-graded layer 3814 thereon aretransparent to the optical radiation of the oscillation wavelength ofthe laser diode of 635 nm. Thus, it is not necessary to remove a part ofthe substrate 3813 or 3814 to provide a passage for the output opticalbeam. Thereby, the fabrication process of the laser diode is simplified.

[0652] In the present embodiment, it is also possible to use otherabsorption-free substrates such as GaInP substrate for the substrate3813. Further, the process of forming the composition-graded layer 3814on the substrate 3813 is not limited to a vapor phase epitaxial process.

[0653] [Thirty-Second Embodiment]

[0654] Next, a vertical cavity laser diode according to a thirty-secondembodiment of the present invention will be described with reference toFIG. 42.

[0655] In the present embodiment, the vertical-cavity laser diodeincludes a pair of DBR structures having a lattice constant between GaAsand GaP, wherein the laser diode is designed to produce the outputoptical beam through the DBR structure located closer to the substratewhile using simultaneously a GaAsP substrate.

[0656] In the present embodiment, a part of the GaAsP substrate isetched away for providing the path of the output optical beam, whereinthe process of etching the GaAsP substrate is facilitated in the presentembodiment by interposing a GaInAsP etching stopper layer between theGaAsP substrate and the DBR structure located closer to the GaPAssubstrate.

[0657] It should be noted that a GaAsP mixed crystal is not transparentto the optical radiation of the wavelength of 630-660 nm when the Ascontent is equal to or larger than 0.63. Thus, there can be a case inwhich the GaAsP substrate absorbs the output laser beam in thevertical-cavity laser diode of the type that emits the output laser beamthrough the DBR structure located closer to the GaAsP substrate,depending on the composition of the GaAsP substrate. Thus, it isnecessary in such a vertical-cavity laser diode to remove a part of theGaAsP substrate for allowing the laser beam to go out withoutabsorption.

[0658] While such an etching of the GaAsP substrate can be achieved byusing a sulfuric acid etchant, the sulfuric acid etchant, reacting upona mixed crystal of AlGaInAsP, can act on the DBR structure depending onthe composition of the DBR structure. On the other hand, a mixed crystalof GaInAsP containing As with a concentration smaller than the Ascontent of the GaAsP substrate, a high selectivity of etching isrealized with respect to the GaAsP substrate. Thus, the presentembodiment uses a GaInAsP mixed crystal layer as an etching stopperlayer in the process of forming an opening in the GaAsP substrate as apassage of the output laser beam. In view of the fact that theselectivity of etching increases with decreasing As content, it ispossible to use a mixed crystal composition of GaInP for the etchingstopper layer.

[0659]FIG. 42 shows the construction of the vertical-cavity laser diodeaccording to the present embodiment.

[0660] Referring to FIG. 42, the vertical-cavity laser diode isconstructed on a substrate 3902 of n-type GaAsP, wherein the laser diodeincludes, on the GaAsP substrate 3902, a buffer layer 3903 of n-typeGaAsP, an etching stopper layer 3912 of n-type GaInP, a DBR structure3904 formed of an alternate repetition of an n-type AlInAsP layer and ann-type AlGaAsP layer, a carrier confinement layer 3905 of undoped GaInP,an active layer 3906 of undoped GaInAsP, another carrier confinementlayer 3907 of undoped GaInP, a DBR structure 3908 formed of an alternaterepetition of a p-type AlInAsP layer and a p-type AlGaAsP layer, a spikeelimination layer 3909 of p-type GaInP, and a contact layer 3910 ofp-type GaAsP, wherein the foregoing layers 3903-3910, including thelayer 3912, are deposited consecutively on the substrate 3902 by anMOCVD process.

[0661] In the present embodiment the n-type GaASP substrate 3902 has acomposition set such that a lattice strain of −1.4% is accumulated withrespect to GaAs. Further, the active layer 3906 of GaInAsP has acomposition that provides a laser oscillation wavelength of 650 nm.Further, it should be noted that the DBR structure 3904 uses AlInAsP forthe low-refractive-index layer and AlGaAsP for the high-refractive-indexlayer.

[0662] After the formation of the foregoing layered structure, aphotolithographic patterning process is conducted in which the DBR 3908,the spike elimination layer 3909 and the contact layer 3910 arepatterned to form a central post structure while using a resist mask.The patterning process is conducted until the GaInP carrier confinementlayer 3907 is exposed.

[0663] After the formation of the central post structure, an SiO₂ film3910 is deposited uniformly by a CVD process, and a photolithographicpatterning process is conducted to form a contact window in the SiO₂film 3910 by using a resist mask so as to expose the GaAsP contact layer3910, and a p-type electrode 3911 is deposited by anevaporation-deposition process.

[0664] In the present embodiment, the bottom surface of the GaAsPsubstrate 3902 is polished and a resist pattern is provided so as toexpose the region aligned with the post structure, and a wet etchingprocess is applied to the GaAsP substrate 3902 while using a sulfuricacid etchant. Thereby, the wet etching process proceeds until the GaInPetching stopper layer 3912 is exposed, wherein the etching stopsspontaneously upon the exposure of the GaInP etching stopper layer 3912due to the selectivity of the GaInP composition. As a result of the wetetching process, an opening is formed in the GaAsP substrate 3902 as theoutput path of the laser beam.

[0665] After the step of forming the opening in the GaAsP substrate3902, an n-type electrode is deposited by an evaporation-depositionprocess on the bottom surface of the GaAsP substrate3902. Further, athermal annealing process is applied to form an ohmic contact at each ofthe electrodes 3901 and 3911.

[0666] By using the etching stopper layer 3912, the etching process forforming the opening in the GaAsP substrate 3902 is controlled exactlyand the laser diode can be produced with little variation.

[0667] As noted previously, a GaInAsP mixed crystal can be used for theetching stopper layer 3912.

[0668] [Thirty-Third Embodiment]

[0669] The laser diodes of the present invention described heretoforewith reference to FIGS. 9-42 can be used for various applications.

[0670]FIG. 43 shows the construction of a xerographic printer that usesthe laser diode according to any of the embodiments of the presentinvention.

[0671] Referring to FIG. 43, the xerographic printer includes a sheetfeed path 4002 including guide rollers 4002 a-4002 d for feeding a sheetfrom a sheet feed stack 4001 one by one to a sheet recovery tray 4003.

[0672] In correspondence to an intermediate location on the sheet feedpath 4002 between the sheet feed stack 4001 and the recovery tray 4003,there is provided a photosensitive medium 4004, which may be aphotosensitive drum or a photosensitive belt, and a laser diode array4005 writes an image to be recorded on the sheet by way of an opticalbeam, wherein the laser diode array may include the visible to redwavelength laser diode described with reference to any of theembodiments of FIGS. 9-42 as an optical source.

[0673] The photosensitive medium 4004 is electrically charged by anelectric charger 4004A, and an electrostatic latent image is formed onthe charged surface of the photosensitive medium 4004 in correspondenceto the part irradiated by the laser beam.

[0674] The electrostatic latent image thus formed on the photosensitivemedium 4004 is developed by toner powers held in a toner cartridge 4006and a toner image is formed on the photosensitive medium 4004 incorrespondence to the toner image. The toner image thus formed on thephotosensitive medium 4004 is then transferred to the sheet on the sheetfeed path 4002 by urging the sheet strongly to the photosensitive medium4004 by an urging roller 4004B.

[0675] The recording sheet thus formed with the toner image is thenfixed by a fixing unit and is forwarded to the sheet recovery tray.

[0676] In the xerographic image recording apparatus, writing of theelectrostatic latent image onto the photosensitive medium 4004 can beachieved by using a red color beam, which is advantageous for high-speedand high-resolution image recording.

[0677] [Thirty-Fourth Embodiment]

[0678]FIG. 44 shows the construction of an optical disk drive accordingto a thirty-fourth embodiment of the present invention.

[0679] Referring to FIG. 44, the optical disk drive includes a rotaryoptical disk 5001 and an optical head 5002 driven by a driving mechanism5003 so as to scan over the surface of the rotary optical disk 5001,wherein the optical head 5002 includes a red-wavelength laser diode 5002a according to any of the embodiments described heretofore withreference to FIGS. 9-42.

[0680] The laser beam produced by the laser diode 5002 a is directed tothe surface of the rotary optical disk 5001 via a lens 5002 a, ahalf-transparent mirror 5002 c and mirrors 5002 d and 5002 e, while theoptical beam reflected by the optical disk 5001 is guided to aphoto-detector 5002 f via the mirrors 5002 e and 5002 d, thehalf-transparent mirror 5002 c and the mirror 5002 g.

[0681] By using the red-wavelength laser diode of the previousembodiments, optical reading and optical writing becomes possible with asmall drive current.

[0682] [Thirty-Fifth Embodiment]

[0683]FIG. 45 shows the construction of an optical module according to athirty-fifth embodiment of the present invention.

[0684] Referring to FIG. 45, the optical module includes, in a modulehousing 6001, a lens 6002 and a laser diode 6003 in optical alignmentwith the lens 6002, wherein the laser diode 6003 may be any of thered-wavelength laser diode described in the previous embodiments withreference to FIGS. 9-42. Further, the optical module includes a plasticoptical fiber 6004 in optical alignment with the lens 6002, and hencethe laser diode 6003. Thereby, the laser beam produced by the laserdiode 6003 is injected into the plastic optical fiber 6004.

[0685] According to the optical module of the present embodiment, alaser beam in the wavelength range of about 650 nm is producedefficiently by using a red-wavelength laser diode explained before,wherein it should be noted that this wavelength of about 650 nmcorresponds to the minimum transmission loss of PMMA which is usedextensively for the material of a plastic optical fiber.

[0686] Thus, the optical module of the present embodiment, and hence thered-wavelength laser diode of the present invention, is expected to playan important role in a low-cost, short-distance optical network.

[0687] While it is illustrated in FIG. 45 that the laser diode 6003 isan edge-emission type laser diode, the vertical-cavity laser diodeexplained with reference to FIGS. 35-42 is also applicable to theoptical module of FIG. 45.

[0688] Further, the present invention is by no means limited to theembodiments described heretofore, but various variations andmodifications may be made without departing from the scope of theinvention.

What is claimed is:
 1. A laser diode, comprising: a substrate of a firstconductivity type, said substrate having a lattice constant of GaAs or alattice constant between GaAs and GaP; a first cladding layer of AlGaInPhaving said first conductivity type formed over said substrate; anactive layer of GaInAsP formed over said first cladding layer; anetching stopper layer of GaInP formed over said active layer; a pair ofcurrent-blocking regions of AlGaInP formed over said etching stopperlayer, said pair of current-blocking regions defining therebetween astrip region; an optical waveguide layer of AlGaInP formed over saidpair of current-blocking regions so as to include said stripe regions,said optical waveguide layer covering said etching stopper layer in saidstripe region; and a second cladding layer of AlGaInP of a secondconductivity type formed over said optical waveguide layer; saidcurrent-blocking regions having an Al content substantially identicalwith an Al content of said second cladding layer.
 2. A laser diode asclaimed in claim 1, wherein said optical waveguide layer has acomposition of GaInP.
 3. A laser diode as claimed in claim 1, whereinsaid substrate comprises a GaAs base substrate of said firstconductivity type, a composition-graded layer of GaAsP of said firstconductivity type formed on said GaAs base substrate, and a GaAsP thickfilm of said first conductivity type formed on said composition-gradedlayer.
 4. A laser diode as claimed in claim 1, wherein each of saidcurrent-blocking regions comprises a stacking of a first AlGaInP layerof said second conductivity type and a second AlGaInP layer of saidfirst conductivity type, said second AlGaInP layer being formed oversaid first AlGaInP layer.
 5. A laser diode as claimed in claim 1,further comprising a cap layer over said current-blocking regions, saidcap layer having a composition of any of GaInP, GaAsP and GaInAsP.
 6. Alaser diode as claimed in claim 1, wherein said stripe region has awidth of 5 μm or less.
 7. A laser diode as claimed in claim 1, whereinat least one of said first cladding layer, said optical waveguide layer,each of said current-blocking regions, and said second cladding layercontain As therein.
 8. A laser diode, comprising: a substrate having alattice constant between GaAs and GaP, said substrate having a firstconductivity type; a first cladding layer of AlGaInP having said firstconductivity type formed over said substrate; a lower optical waveguidelayer of GaInP formed over said first cladding layer; an active layer ofGaInAsP formed over said lower optical waveguide layer; a first upperoptical waveguide layer of GaInP formed over said active layer; a pairof current-blocking regions of AlGaInP formed over said first upperoptical waveguide layer, said pair of current-blocking regions definingtherebetween a stripe region; a second upper optical waveguide layer ofGaInP formed over said pair of current-blocking regions so as to includesaid stripe regions, said second upper optical waveguide layer coveringsaid first upper optical waveguide layer in said stripe region; and asecond cladding layer of AlGaInP having a second conductivity typeformed over said second upper optical waveguide layer: saidcurrent-blocking regions having an Al content generally identical withan Al content of said second cladding layer.
 9. A laser diode as claimedin claim 8, wherein said optical waveguide layer has a composition ofGaInP.
 10. A laser diode as claimed in claim 8, wherein said substratecomprises a GaAs base substrate of said first conductivity type, acomposition-graded layer of GaAsP of said first conductivity type formedon said GaAs base substrate, and a GaAsP thick film of said firstconductivity type formed on said composition-graded layer.
 11. A laserdiode as claimed in claim 8, wherein each of said current-blockingregions comprises a stacking of a first AlGaInP layer of said secondconductivity type and a second AlGaInP layer of said first conductivitytype, said second AlGaInP layer being formed over said first AlGaInPlayer.
 12. A laser diode as claimed in claim 8, further comprising a caplayer over said current-blocking regions, said cap layer having acomposition of any of GaInP, GaAsP and GaInAsP.
 13. A laser diode asclaimed in claim 8, wherein said stripe region has a width of 5 μm orless.
 14. A laser diode as claimed in claim 8, wherein at least one ofsaid first cladding layer, said lower optical waveguide layer, saidfirst upper optical waveguide layer, said second upper optical waveguidelayer, said current-blocking regions, and said second cladding layercontains As therein.
 15. A laser diode, comprising: a substrate having alattice constant between GaAs and GaP, said substrate having a firstconductivity type; a first cladding layer of AlGaInP having said firstconductivity type formed over said substrate; a lower optical waveguidelayer of GaInP formed over said first cladding layer; an active layer ofGaInAsP formed over said lower optical waveguide layer; a first upperoptical waveguide layer formed over said active layer; a pair ofcurrent-blocking regions of AlGaInP formed over said first upper opticalwaveguide layer, said pair of current-blocking regions definingtherebetween a stripe region; a second upper optical waveguide layer ofGaInP formed over said pair of current-blocking regions so as to includesaid stripe regions, said second upper optical waveguide layer coveringsaid first upper optical waveguide layer in said stripe region; and asecond cladding layer of AlGaInP having a second conductivity typeformed over said second upper optical waveguide layer; saidcurrent-blocking regions having an Al content generally identical withan Al content of said second cladding layer, said first upper opticalwaveguide layer of GaInP and said second upper optical waveguide layerof GaInP having respective thicknesses such that a sum of said thicknessof said first upper optical waveguide layer and said thickness of saidsecond upper optical waveguide layer is equal to a thickness of saidlower optical waveguide layer of GaInP.
 16. A laser diode as claimed inclaim 15, wherein said substrate comprises a GaAs base substrate of saidfirst conductivity type, a composition-graded layer of GaAsP of saidfirst conductivity type formed on said GaAs base substrate, and a GaAsPthick film of said first conductivity type formed on saidcomposition-graded layer.
 17. A laser diode as claimed in claim 15,wherein each of said current-blocking regions comprises a stacking of afirst AlGaInP layer of said second conductivity type and a secondAlGaInP layer of said first conductivity type, said second AlGaInP layerbeing formed over said first AlGaInP layer.
 18. A laser diode as claimedin claim 15, further comprising a cap layer over said current-blockingregions, said cap layer having a composition of any of GaInP, GaAsP andGaInAsP.
 19. A laser diode as claimed in claim 15, wherein said striperegion has a width of 5 μm or less.
 20. A laser diode as claimed inclaim 15, wherein at least one of said first cladding layer, said loweroptical waveguide layer, said first upper optical waveguide layer, saidsecond upper optical waveguide layer, each of said current-blockingregion, and said second cladding layer contains As therein.
 21. A laserdiode, comprising: a substrate having a first conductivity type; a firstcladding layer of said first conductivity type provided over saidsubstrate, said first cladding layer having a lattice constant betweenGaAs and GaP; an active layer formed over said first cladding layer; asecond cladding layer of a second conductivity type provided over saidactive layer, said second cladding layer having said lattice constant; aridge-stripe region formed in said second cladding layer; and a pair ofcurrent-blocking regions of said first conductivity type respectivelyprovided over said second cladding layer at both lateral sides of saidridge-stripe region; each of said current-blocking regions having acomposition represented as(Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0≦y₁≦1,0<z₁≦1).
 22. A laser diode as claimed in claim 21, wherein saidcurrent-blocking regions have a composition represented as(Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0≦y₁≦1,0.01≦z₁≦1).
 23. A laser diode as claimed in claim 21, wherein saidcurrent-blocking regions are transparent with respect to a laser beamproduced by said laser diode.
 24. A laser diode as claimed in claim 21,wherein said second cladding layer includes therein a layer having acomposition represented as Ga_(a1)In_(1-a1)As_(b1)P_(1-b1) (0≦a₁≦1,0≦b₁≦1).
 25. A laser diode as claimed in claim 21, further comprising anoptical waveguide layer of GaInP provided adjacent to said active layerin correspondence to at least one of an interface between said activelayer and said first cladding layer and an interface between said activelayer and said second cladding layer.
 26. A laser diode, comprising: asubstrate having a first conductivity type; a first cladding layer ofsaid first conductivity type provided over said substrate, said firstcladding layer having a lattice constant between GaAs and GaP; an activelayer formed over said first cladding layer; a second cladding layer ofa second conductivity type provided over said active layer, said secondcladding layer having said lattice constant; a current-blocking layer ofsaid first conductivity type respectively provided over said secondcladding layer; a stripe depression formed in said current-blockinglayer; and a third cladding layer of said second conductivity typeformed over said current-blocking layer so as to include said stripedepression, said current-blocking layer having a composition representedas (Al_(x2)Ga_(1-x2))_(y2)In_(1-y2)As_(z2)P_(1-z2) (0≦x₂≦1, 0≦y₂≦1,0<z₂≦1).
 27. A laser diode as claimed in claim 26, wherein saidcurrent-blocking layer having a composition represented as(Al_(x2)Ga_(1-x2))_(y2)In_(1-y2)As_(z2)P_(1-z2) (0≦x₂≦1, 0≦y₂≦1,0.01≦z₂≦1).
 28. A laser diode as claimed in claim 26, wherein saidcurrent-blocking layer is transparent with respect to a laser beamproduced by said laser diode.
 29. A laser diode as claimed in claim 28,further comprising a layer having a composition represented asGa_(a2)In_(1-a2)As_(b2)P_(1-b2) (0≦a₂≦1, 0≦b₂≦1) on said second claddinglayer.
 30. A laser diode as claimed in claim 28, further comprising alayer having a composition represented asGa_(a3)In_(1-a3)As_(b3)P_(1-b3) (0≦a₃≦1, 0≦b₃≦1) on saidcurrent-blocking layer.
 31. A laser diode as claimed in claim 26,further comprising an optical waveguide layer of GaInP provided adjacentto said active layer in correspondence to at least one of an interfacebetween said active layer and said first cladding layer and an interfacebetween said active layer and said second cladding layer.
 32. Asemiconductor light-emitting device, comprising: a semiconductorsubstrate; an active layer provided over said semiconductor substrate,said active layer emitting optical radiation; a semiconductor layervertically sandwiching said active layer with another semiconductorlayer, said semiconductor layer having a bandgap larger than a bandgapof said active layer and a lattice constant between GaP and GaAs, saidsemiconductor layer containing a to be-oxidized layer in a part thereofwith a composition represented as Al_(x)Ga_(y)In_(l-x-y)P_(t)As_(1-t)(0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), a part of said to be-oxidized layer beingoxidized to form a selective oxidation region.
 33. A semiconductorlight-emitting device, as claimed in claim 32, wherein saidsemiconductor substrate is formed of a material of GaPAs, and whereinsaid to-be-oxidized layer has a composition achieving a lattice matchingwith said semiconductor substrate.
 34. A semiconductor light-emittingdevice as claimed in claim 32, wherein said to-be-oxidized layer has acomposition represented as AlP_(t)As_(1-t) (0≦t≦1), containing Al as theonly group III element.
 35. A semiconductor light-emitting device asclaimed in claim 32, wherein said to-be-oxidized layer has asuperlattice structure in which an AlAs layer and a layer having alattice constant between GaP and GaAs are stacked a plurality of times.36. A semiconductor light-emitting device as claimed in claim 35,wherein said layer having a lattice constant between GaP and GaAs isAlPAs.
 37. A semiconductor light-emitting device as claimed in claim 32,wherein said to-be-oxidized layer is formed with said selectiveoxidation region with a width w2, with a not-oxidized region formed witha width w1, wherein said not-oxidized region is formed such that a ratiow1 to a sum of said width w1 and said width w2 is less than 0.6.
 38. Asemiconductor light-emitting device as claimed in claim 32, wherein saidsemiconductor light-emitting device includes a ridge structure formed ina part of said semiconductor layer located above said to-be-oxidizedlayer, said ridge structure having a ridge width exceeding 10 μm.
 39. Asemiconductor light-emitting device as claimed in claim 32, wherein saidsemiconductor light-emitting device includes a ridge structure formed ina part of said semiconductor layer located above said to-be-oxidizedlayer, and wherein an etching stopper layer having a compositionGa_(y)In_(1-y)P_(t)As_(1-t) (0<y≦1, 0≦t≦1) is provided under saidto-be-oxidized layer.
 40. A semiconductor light-emitting device,comprising: a semiconductor substrate; an active layer provided oversaid semiconductor substrate, said active layer producing opticalradiation; and a pair of cladding layers sandwiching said active layervertically, said active layer being one of a single quantum wellstructure containing therein a quantum well layer and a multiple quantumwell structure containing therein a quantum well layer and a barrierlayer, said quantum well layer comprising a mixed crystal of AlGaInPAshaving a composition represented as(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x₁<1, 0<α₁≦1,0≦t₁≦1), said barrier layer comprising a mixed crystal of AlGaInPAshaving a composition represented as(Al_(x2)Ga_(1-x2))_(α2)In_(1-α2)P_(t2)As_(1-t2) (0≦x₂<1, 0.5<α₂<1,0≦t₂≦1), each of said cladding layers comprising a mixed crystal ofAlGaInPAs containing Al and having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1), eachof said cladding layers having a lattice constant between GaP and GaAsand a bandgap larger than a bandgap of said active layer, an opticalwaveguide layer of AlGaInPAs interposed between said active layer andeach of said cladding layers, said optical waveguide layer having abandgap larger than the bandgap of said active layer but smaller thanthe bandgap of said cladding layer, said optical waveguide layer havinga composition represented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u)(0≦z<1, 0.5<γ<1, 0<u≦1), a to-be-oxidized layer provided in at least oneof said cladding layers such that said cladding layer contains saidto-be-oxidized layer in correspondence to a part thereof, or betweensaid active layer and one of said cladding layers, said to-be-oxidizedlayer having a composition represented asAl_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t) (0.8≦x≦1, 0≦y≦0.2, 0≦t≦1), a part ofsaid to-be-oxidized layer being selectively oxidized to form a selectiveoxidized region.
 41. A semiconductor light-emitting device, as claimedin claim 40, wherein said semiconductor substrate is formed of amaterial of GaPAs, and wherein said to-be-oxidized layer has acomposition achieving a lattice matching with said semiconductorsubstrate.
 42. A semiconductor light-emitting device as claimed in claim41, wherein said to-be-oxidized layer has a composition represented asAlP_(t)As_(1-t) (0≦t≦1), containing Al as the only group III element.43. A semiconductor light-emitting device as claimed in claim 40,wherein said to-be-oxidized layer has a superlattice structure in whichan AlAs layer and a layer having a lattice constant between GaP and GaAsare stacked a plurality of times.
 44. A semiconductor light-emittingdevice as claimed in claim 43, wherein said layer having a latticeconstant between GaP and GaAs is AlPAs.
 45. A semiconductorlight-emitting device as claimed in claim 40, wherein saidto-be-oxidized layer is formed with said selective oxidation region witha width w2, with a not-oxidized region formed with a width w1, whereinsaid not-oxidized region is formed such that a ratio w1 to a sum of saidwidth w1 and said width w2 is less than 0.6.
 46. A semiconductorlight-emitting device as claimed in claim 40, wherein said semiconductorlight-emitting device includes a ridge structure formed in a part ofsaid cladding layer located above said to-be-oxidized layer, said ridgestructure having a ridge width exceeding 10 μm.
 47. A semiconductorlight-emitting device as claimed in claim 40, wherein said semiconductorlight-emitting device includes a ridge structure formed in a part ofsaid cladding layer located above said to-be-oxidized layer, and whereinan etching stopper layer having a compositionGa_(y)In_(1-y)P_(t)As_(1-t) (0<y≦1, 0≦t≦1) is provided under saidto-be-oxidized layer.
 48. A semiconductor light-emitting device,comprising: a GaAs substrate; an active layer provided over said GaAssubstrate, said active layer emitting an optical radiation; a pair ofsemiconductor layers sandwiching said active layer vertically, saidsemiconductor layer containing a to be-oxidized layer in a part thereofwith a composition represented as Al_(x)Ga_(y)In_(1-x-y)P_(t)As_(1-t)(0.8≦x≦1, 0≦y≦0.2, 0≦t≦1) and containing P, a part of said tobe-oxidized layer being oxidized to form a selective oxidation region.49. A semiconductor light-emitting device, comprising: a GaAs substrate;an active layer of an AlGaInP system formed over said GaAs substrate,said active layer emitting optical radiation; a pair of semiconductorlayers sandwiching said active layer vertically, each of saidsemiconductor layers having a bandgap larger than a bandgap of saidactive layer, said semiconductor layers including, in a part thereof, alayer of AlGaInAs having a composition represented asAl_(x)Ga_(y)In_(1-y)As (0.8≦x≦1, 0≦y≦0.2), a part of said semiconductorlayer being oxidized to form a pair of oxidized regions, with anot-oxidized region formed therebetween with a width w1, a total widthof said pair of oxidized regions being defined as w2, wherein said widthw1 is set such that a ratio of said width w1 with respect to a sum ofsaid width w1 and said width w2, defined as w1/(w1+w2) is smaller than0.6.
 50. A semiconductor light-emitting device, comprising: a GaAssubstrate; an active layer provided over said GaAs substrate, saidactive layer emitting optical radiation; and a pair of semiconductorlayers sandwiching said active layer vertically, each of saidsemiconductor layers having a bandgap larger than a bandgap of saidactive layer, said semiconductor layers including, in a part thereof, alayer of AlGaInAs having a composition represented asAl_(x)Ga_(y)In_(1-x-y)As (0.8≦x≦1, 0≦y≦0.2), a part of saidsemiconductor layer being oxidized to form an oxidized region, a ridgestructure being formed in a part of said semiconductor layer located atleast above said layer of AlGaInAs, said ridge structure having a ridgewidth exceeding 10 μm.
 51. A vertical-cavity laser diode, comprising: asubstrate; an active layer provided over said substrate, said activelayer emitting optical radiation; and a distributed Bragg reflectorprovided over said substrate in an optical path of said opticalradiation emitted from said active layer in a direction perpendicularlyto a plane of said active layer, said distributed Bragg reflectorcomprising a plurality of layers stacked over said substrate, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP and including at least two semiconductor layers of respective,mutually different compositions, at least one of said semiconductorlayers having a composition represented as(Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z) (0≦x₁≦1, 0.5≦y₁≦1,0<z₁<1).
 52. A vertical-cavity laser diode as claimed in claim 51,wherein said substrate carries an optical window at a bottom surfacethereof.
 53. A vertical-cavity laser diode as claimed in claim 51,wherein said substrate has a composition of GaAsP, and wherein saidvertical-cavity laser diode further includes a GaInAsP layer betweensaid substrate and said distributed Bragg reflector.
 54. Avertical-cavity laser diode as claimed in claim 51, wherein saidsubstrate has a composition of GaAsP, and wherein said vertical-cavitylaser diode includes a GaInP layer between said semiconductor substrateand said distributed Bragg reflector.
 55. A vertical-cavity laser diode,comprising: a substrate; an active layer provided over said substrate,said active layer emitting optical radiation; and a distributed Braggreflector provided over said substrate in an optical path of saidoptical radiation emitted from said active layer in a directionperpendicularly to a plane of said active layer, said distributed Braggreflector comprising a plurality of layers stacked over said substrate,said active layer having a composition represented asGa_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1).
 56. Avertical-cavity laser diode as claimed in claim 55, wherein saidsubstrate carries an optical window at a bottom surface thereof.
 57. Avertical-cavity laser diode as claimed in claim 55, wherein saidsubstrate has a composition of GaAsP, and wherein said vertical-cavitylaser diode further includes a GaInAsP layer between said substrate andsaid distributed Bragg reflector.
 58. A vertical-cavity laser diode asclaimed in claim 55, wherein said substrate has a composition of GaAsP,and wherein said vertical-cavity laser diode includes a GaInP layerbetween said semiconductor substrate and said distributed Braggreflector.
 59. A vertical-cavity laser diode, comprising: a substrate;an active layer provided over said substrate, said active layer emittingoptical radiation; a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP; and a pair of semiconductor layers having acomposition represented as Ga_(y3)In_(1-y3)P (0.5≦y₃≦1) provided atupper and lower sides of said active layer.
 60. A vertical-cavity laserdiode as claimed in claim 59, wherein said substrate carries an opticalwindow at a bottom surface thereof.
 61. A vertical-cavity laser diode asis claimed in claim 59, wherein said substrate has a composition ofGaAsP, and wherein said vertical-cavity laser diode further includes aGaInAsP layer between said substrate and said distributed Braggreflector.
 62. A vertical-cavity laser diode as claimed in claim 59,wherein said substrate has a composition of GaAsP, and wherein saidvertical-cavity laser diode includes a GaInP layer between saidsemiconductor substrate and said distributed Bragg reflector.
 63. Avertical-cavity laser diode, comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted from said active layerin a direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP; a contact layer provided over said distributed Bragg reflector; andan electrode provided on said contact layer in ohmic contact therewith,said contact layer being transparent to an optical beam produced as aresult of interaction of said optical radiation produced by said activelayer with said distributed Bragg reflector.
 64. A vertical-cavity laserdiode as claimed in claim 63, wherein said substrate carries an opticalwindow at a bottom surface thereof.
 65. A vertical-cavity laser diode asclaimed in claim 63, wherein said substrate has a composition of GaAsP,and wherein said vertical-cavity laser diode further includes a GaInAsPlayer between said substrate and said distributed Bragg reflector.
 66. Avertical-cavity laser diode as claimed in claim 63, wherein saidsubstrate has a composition of GaAsP, and wherein said vertical-cavitylaser diode includes a GaInP layer between said semiconductor substrateand said distributed Bragg reflector.
 67. A vertical-cavity laser diode,comprising: a substrate; an active layer provided over said substrate,said active layer emitting optical radiation; and a distributed Braggreflector provided over said substrate in an optical path of saidoptical radiation emitted from said active layer in a directionperpendicularly to a plane of said active layer, said distributed Braggreflector having a lattice constant between GaAs and GaP, saiddistributed Bragg reflector including therein a semiconductor layerhaving a composition represented as AlAs_(z4)P_(1-z4) (0≦z₄≦1).
 68. Avertical-cavity laser diode as claimed in claim 67, wherein saidsubstrate carries an optical window at a bottom surface thereof.
 69. Avertical-cavity laser diode as claimed in claim 67, wherein saidsubstrate has a composition of GaAsP, and wherein said vertical-cavitylaser diode further includes a GaInAsP layer between said substrate andsaid distributed Bragg reflector.
 70. A vertical-cavity laser diode asclaimed in claim 67, wherein said substrate has a composition of GaAsP,and wherein said vertical-cavity laser diode includes a GaInP layerbetween said semiconductor substrate and said distributed Braggreflector.
 71. A vertical-cavity laser diode, comprising: a substrate;an active layer provided over said substrate, said active layer emittingoptical radiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP, said distributed Bragg reflector including thereina semiconductor layer having a composition represented asAlAs_(z5)P_(1-z5) (0≦z₅≦1), said semiconductor layer being laterallysandwiched by a pair of oxide regions formed coplanar to saidsemiconductor layer, said semiconductor layer and said pair of oxideregions forming a current confinement structure.
 72. A vertical-cavitylaser diode as claimed in claim 71, wherein said substrate carries anoptical window at a bottom surface thereof.
 73. A vertical-cavity laserdiode as claimed in claim 71, wherein said substrate has a compositionof GaAsP, and wherein said vertical-cavity laser diode further includesa GaInAsP layer between said substrate and said distributed Braggreflector.
 74. A vertical-cavity laser diode as claimed in claim 71,wherein said substrate has a composition of GaAsP, and wherein saidvertical-cavity laser diode includes a GaInP layer between saidsemiconductor substrate and said distributed Bragg reflector.
 75. Avertical-cavity laser diode, comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted perpendicularly to aplane of said active layer, said distributed Bragg reflector having alattice constant between GaAs and GaP; and a semiconductor layerinterposed between said active layer and said distributed Braggreflector, said semiconductor layer having a composition represented asAlAs_(z6)P_(1-z6) (0≦z₆≦1), said semiconductor layer being laterallysandwiched by a pair of oxidized regions formed coplanar to saidsemiconductor layer.
 76. A vertical-cavity laser diode as claimed inclaim 75, wherein said substrate carries an optical window at a bottomsurface thereof.
 77. A vertical-cavity laser diode as claimed in claim75, wherein said substrate has a composition of GaAsP, and wherein saidvertical-cavity laser diode further includes a GaInAsP layer betweensaid substrate and said distributed Bragg reflector.
 78. Avertical-cavity laser diode as claimed in claim 75, wherein saidsubstrate has a composition of GaAsP, and wherein said vertical-cavitylaser diode includes a GaInP layer between said semiconductor substrateand said distributed Bragg reflector.
 79. A xerographic image recordingapparatus, comprising: a sheet feeding mechanism feeding a sheet from asheet stocker to a sheet recovery tray along a sheet feeding path; aphotosensitive medium provided in said sheet feeding path in contactwith said sheet transported along said photosensitive medium; an opticalunit recording an electrostatic latent image on said photosensitivemedium by exposing said photosensitive medium by an optical beam, atoner unit supplying toner to said photosensitive medium so as todevelop said electrostatic latent image to form a toner image; and afixing unit provided adjacent to said sheet feed path, said fixing unitfixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector comprising a plurality of layersstacked over said substrate, said distributed Bragg reflector having alattice constant between GaAs and GaP and including at least twosemiconductor layers of respective, mutually different compositions, atleast one of said semiconductor layers having a composition representedas (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0.5≦y₁≦1,0<z₁<1).
 80. A xerographic image recording apparatus, comprising: asheet feeding mechanism feeding a sheet from a sheet stocker to a sheetrecovery tray along a sheet feeding path; a photosensitive mediumprovided in said sheet feeding path in contact with said sheettransported along said photosensitive medium; an optical unit recordingan electrostatic latent image on said photosensitive medium by exposingsaid photosensitive medium by an optical beam, a toner unit supplyingtoner to said photosensitive medium so as to develop said electrostaticlatent image to form a toner image; and. a fixing unit provided adjacentto said sheet feed path, said fixing unit fixing said toner on saidsheet, said optical unit comprising a vertical-cavity laser diode forproducing said optical beam, said vertical-cavity laser diodecomprising: a substrate; an active layer provided over said substrate,said active layer emitting optical radiation; and a distributed Braggreflector provided over said substrate in an optical path of saidoptical radiation emitted from said active layer in a directionperpendicularly to a plane of said active layer, said distributed Braggreflector comprising a plurality of layers stacked over said substrate,said active layer having a composition represented asGa_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1).
 81. A xerographicimage recording apparatus, comprising: a sheet feeding mechanism feedinga sheet from a sheet stocker to a sheet recovery tray along a sheetfeeding path; a photosensitive medium provided in said sheet feedingpath in contact with said sheet transported along said photosensitivemedium; an optical unit recording an electrostatic latent image on saidphotosensitive medium by exposing said photosensitive medium by anoptical beam, a toner unit supplying toner to said photosensitive mediumso as to develop said electrostatic latent image to form a toner image;and a fixing unit provided adjacent to said sheet feed path, said fixingunit fixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector comprising a plurality of layersstacked over said substrate, said active layer having a compositionrepresented as Ga_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1).
 82. Axerographic image recording apparatus, comprising: a sheet feedingmechanism feeding a sheet from a sheet stocker to a sheet recovery trayalong a sheet feeding path; a photosensitive medium provided in saidsheet feeding path in contact with said sheet transported along saidphotosensitive medium; an optical unit recording an electrostatic latentimage on said photosensitive medium by exposing said photosensitivemedium by an optical beam, a toner unit supplying toner to saidphotosensitive medium so as to develop said electrostatic latent imageto form a toner image; and a fixing unit provided adjacent to said sheetfeed path, said fixing unit fixing said toner on said sheet, saidoptical unit comprising a vertical-cavity laser diode for producing saidoptical beam, said vertical-cavity laser diode comprising: a substrate;an active layer provided over said substrate, said active layer emittingoptical radiation; a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP; and a pair of semiconductor layers having acomposition represented as Ga_(y3)In_(1-y3)P (0.5≦y₃≦1) provided atupper and lower sides of said active layer.
 83. A xerographic imagerecording apparatus, comprising: a sheet feeding mechanism feeding asheet from a sheet stocker to a sheet recovery tray along a sheetfeeding path; a photosensitive medium provided in said sheet feedingpath in contact with said sheet transported along said photosensitivemedium; an optical unit recording an electrostatic latent image on saidphotosensitive medium by exposing said photosensitive medium by anoptical beam, a toner unit supplying toner to said photosensitive mediumso as to develop said electrostatic latent image to form a toner image;and a fixing unit provided adjacent to said sheet feed path, said fixingunit fixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted from said active layerin a direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP; a contact layer provided over said distributed Bragg reflector; andan electrode provided on said contact layer in ohmic contact therewith,said contact layer being transparent to an optical beam produced as aresult of interaction of said optical radiation produced by said activelayer with said distributed Bragg reflector.
 84. A xerographic imagerecording apparatus, comprising: a sheet feeding mechanism feeding asheet from a sheet stocker to a sheet recovery tray along a sheetfeeding path; a photosensitive medium provided in said sheet feedingpath in contact with said sheet transported along said photosensitivemedium; an optical unit recording an electrostatic latent image on saidphotosensitive medium by exposing said photosensitive medium by anoptical beam, a toner unit supplying toner to said photosensitive mediumso as to develop said electrostatic latent image to form a toner image;and a fixing unit provided adjacent to said sheet feed path, said fixingunit fixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP, said distributed Bragg reflector including thereina semiconductor layer having a composition represented asAlAs_(z4)P_(1-z4) (0≦z₄≦1).
 85. A xerographic image recording apparatus,comprising: a sheet feeding mechanism feeding a sheet from a sheetstocker to a sheet recovery tray along a sheet feeding path; aphotosensitive medium provided in said sheet feeding path in contactwith said sheet transported along said photosensitive medium; an opticalunit recording an electrostatic latent image on said photosensitivemedium by exposing said photosensitive medium by an optical beam, atoner unit supplying toner to said photosensitive medium so as todevelop said electrostatic latent image to form a toner image; and afixing unit provided adjacent to said sheet feed path, said fixing unitfixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP, said distributed Bragg reflector including thereina semiconductor layer having a composition represented asAlAs_(z5)P_(1-z5) (0≦Z₅≦1), said semiconductor layer being laterallysandwiched by a pair of oxide regions formed coplanar to saidsemiconductor layer, said semiconductor layer and said pair of oxideregions forming a current confinement structure.
 86. A xerographic imagerecording apparatus, comprising: a sheet feeding mechanism feeding asheet from a sheet stocker to a sheet recovery tray along a sheetfeeding path; a photosensitive medium provided in said sheet feedingpath in contact with said sheet transported along said photosensitivemedium; an optical unit recording an electrostatic latent image on saidphotosensitive medium by exposing said photosensitive medium by anoptical beam, a toner unit supplying toner to said photosensitive mediumso as to develop said electrostatic latent image to form a toner image;and a fixing unit provided adjacent to said sheet feed path, said fixingunit fixing said toner on said sheet, said optical unit comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted perpendicularly to aplane of said active layer, said distributed Bragg reflector having alattice constant between GaAs and GaP; and a semiconductor layerinterposed between said active layer and said distributed Braggreflector, said semiconductor layer having a composition represented asAlAs_(z6)P_(1-z6) (0≦z₆≦1), said semiconductor layer being laterallysandwiched by a pair of oxidized regions formed coplanar to saidsemiconductor layer.
 87. An optical information recording apparatus,comprising: a recording medium carrying optical information; and anoptical source scanning said recording medium by an optical beam, saidoptical source comprising a vertical-cavity laser diode for producingsaid optical beam, said vertical-cavity laser diode comprising: asubstrate; an active layer provided over said substrate, said activelayer emitting optical radiation; and a distributed Bragg reflectorprovided over said substrate in an optical path of said opticalradiation emitted from said active layer in a direction perpendicularlyto a plane of said active layer, said distributed Bragg reflectorcomprising a plurality of layers stacked over said substrate, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP and including at least two semiconductor layers of respective,mutually different compositions, at least one of said semiconductorlayers having a composition represented as(Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0.5≦y₁≦1,0<z₁<1).
 88. An optical information recording apparatus, comprising: arecording medium carrying optical information; and an optical sourcescanning said recording medium by an optical beam, said optical sourcecomprising a vertical-cavity laser diode for producing said opticalbeam, said vertical-cavity laser diode comprising: a substrate; anactive layer provided over said substrate, said active layer emittingoptical radiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector comprising a plurality of layersstacked over said substrate, said active layer having a compositionrepresented as Ga_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1). 89.An optical information recording apparatus, comprising: a recordingmedium carrying optical information; and an optical source scanning saidrecording medium by an optical beam, said optical source comprising avertical-cavity laser diode for producing said optical beam, saidvertical-cavity laser diode comprising: a substrate; an active layerprovided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted from said active layerin a direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP; and a pair of semiconductor layers having a composition representedas Ga_(y3)In_(1-y3)P (0.5≦y₃≦1) provided at upper and lower sides ofsaid active layer.
 90. An optical information recording apparatus,comprising: a recording medium carrying optical information; and anoptical source scanning said recording medium by an optical beam, saidoptical source comprising a vertical-cavity laser diode for producingsaid optical beam, said vertical-cavity laser diode comprising: asubstrate; an active layer provided over said substrate, said activelayer emitting optical radiation; a distributed Bragg reflector providedover said substrate in an optical path of said optical radiation emittedfrom said active layer in a direction perpendicularly to a plane of saidactive layer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP; a contact layer provided over said distributedBragg reflector; and an electrode provided on said contact layer inohmic contact therewith, said contact layer being transparent to anoptical beam produced as a result of interaction of said opticalradiation produced by said active layer with said distributed Braggreflector.
 91. An optical information recording apparatus, comprising: arecording medium carrying optical information; and an optical sourcescanning said recording medium by an optical beam, said optical sourcecomprising a vertical-cavity laser diode for producing said opticalbeam, said vertical-cavity laser diode comprising: a substrate; anactive layer provided over said substrate, said active layer emittingoptical radiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP, said distributed Bragg reflector including thereina semiconductor layer having a composition represented asAlAs_(z4)P_(1-z4) (0≦z₄≦1).
 92. An optical information recordingapparatus, comprising: a recording medium carrying optical information;and an optical source scanning said recording medium by an optical beam,said optical source comprising a vertical-cavity laser diode forproducing said optical beam, said vertical-cavity laser diodecomprising: a substrate; an active layer provided over said substrate,said active layer emitting optical radiation; and a distributed Braggreflector provided over said substrate in an optical path of saidoptical radiation emitted from said active layer in a directionperpendicularly to a plane of said active layer, said distributed Braggreflector having a lattice constant between GaAs and GaP, saiddistributed Bragg reflector including therein a semiconductor layerhaving a composition represented as AlAs_(z5)P_(1-z5) (0≦z₅≦1), saidsemiconductor layer being laterally sandwiched by a pair of oxideregions formed coplanar to said semiconductor layer, said semiconductorlayer and said pair of oxide regions forming a current confinementstructure.
 93. An optical information recording apparatus, comprising: arecording medium carrying optical information; and an optical sourcescanning said recording medium by an optical beam, said optical sourcecomprising a vertical-cavity laser diode for producing said opticalbeam, said vertical-cavity laser diode comprising: a substrate; anactive layer provided over said substrate, said active layer emittingoptical radiation; a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emittedperpendicularly to a plane of said active layer, said distributed Braggreflector having a lattice constant between GaAs and GaP; and asemiconductor layer interposed between said active layer and saiddistributed Bragg reflector, said semiconductor layer having acomposition represented as AlAs_(z6)P_(1-z6) (0≦z₆≦1), saidsemiconductor layer being laterally sandwiched by a pair of oxidizedregions formed coplanar to said semiconductor layer.
 94. An opticaltelecommunication system, comprising: an optical fiber; and avertical-cavity laser diode optically coupled to said optical fiber,said vertical-cavity laser diode comprising: a substrate; an activelayer provided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector comprising a plurality of layersstacked over said substrate, said distributed Bragg reflector having alattice constant between GaAs and GaP and including at least twosemiconductor layers of respective, mutually different compositions, atleast one of said semiconductor layers having a composition representedas (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)As_(z1)P_(1-z1) (0≦x₁≦1, 0.5≦y₁≦1,0<z₁<1).
 95. An optical telecommunication system, comprising: an opticalfiber; and a vertical-cavity laser diode optically coupled to saidoptical fiber, said vertical-cavity laser diode comprising: a substrate;an active layer provided over said substrate, said active layer emittingoptical radiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector comprising a plurality of layersstacked over said substrate, said active layer having a compositionrepresented as Ga_(1-x2)In_(1-y2)As_(z2)P_(1-z2) (0≦y₂≦1, 0≦z₂≦1). 96.An optical telecommunication system, comprising: an optical fiber; and avertical-cavity laser diode optically coupled to said optical fiber,said vertical-cavity laser diode comprising: a substrate; an activelayer provided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted from said active layerin a direction perpendicularly to a plane of said active layer, saiddistributed Bragg reflector having a lattice constant between GaAs andGaP; and a pair of semiconductor layers having a composition representedas Ga_(y3)In_(1-y3)P (0.5≦y₃≦1) provided at upper and lower sides ofsaid active layer.
 97. An optical telecommunication system, comprising:an optical fiber; and a vertical-cavity laser diode optically coupled tosaid optical fiber, said vertical-cavity laser diode comprising: asubstrate; an active layer provided over said substrate, said activelayer emitting optical radiation; a distributed Bragg reflector providedover said substrate in an optical path of said optical radiation emittedfrom said active layer in a direction perpendicularly to a plane of saidactive layer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP; a contact layer provided over said distributedBragg reflector; and an electrode provided on said contact layer inohmic contact therewith, said contact layer being transparent to anoptical beam produced as a result of interaction of said opticalradiation produced by said active layer with said distributed Braggreflector.
 98. An optical telecommunication system, comprising: anoptical fiber; and a vertical-cavity laser diode optically coupled tosaid optical fiber, said vertical-cavity laser diode comprising: asubstrate; an active layer provided over said substrate, said activelayer emitting optical radiation; and a distributed Bragg reflectorprovided over said substrate in an optical path of said opticalradiation emitted from said active layer in a direction perpendicularlyto a plane of said active layer, said distributed Bragg reflector havinga lattice constant between GaAs and GaP, said distributed Braggreflector including therein a semiconductor layer having a compositionrepresented as AlAs_(z4)P_(1-z4) (0≦z₄≦1).
 99. An opticaltelecommunication system, comprising: an optical fiber; and avertical-cavity laser diode optically coupled to said optical fiber,said vertical-cavity laser diode comprising: a substrate; an activelayer provided over said substrate, said active layer emitting opticalradiation; and a distributed Bragg reflector provided over saidsubstrate in an optical path of said optical radiation emitted from saidactive layer in a direction perpendicularly to a plane of said activelayer, said distributed Bragg reflector having a lattice constantbetween GaAs and GaP, said distributed Bragg reflector including thereina semiconductor layer having a composition represented asAlAs_(z5)P_(1-z5) (0≦z₅≦1), said semiconductor layer being laterallysandwiched by a pair of oxide regions formed coplanar to saidsemiconductor layer, said semiconductor layer and said pair of oxideregions forming a current confinement structure.
 100. An opticaltelecommunication system, comprising: an optical fiber; and avertical-cavity laser diode optically coupled to said optical fiber,said vertical-cavity laser diode comprising: a substrate; an activelayer provided over said substrate, said active layer emitting opticalradiation; a distributed Bragg reflector provided over said substrate inan optical path of said optical radiation emitted perpendicularly to aplane of said active layer, said distributed Bragg reflector having alattice constant between GaAs and GaP; and a semiconductor layerinterposed between said active layer and said distributed Braggreflector, said semiconductor layer having a composition represented asAlAs_(z6)P_(1-z6) (0≦z₆≦1), said semiconductor layer being laterallysandwiched by a pair of oxidized regions formed coplanar to saidsemiconductor layer.